Hmm. Pup is bouncing around being vocal tonight, so will contribute here: He has some brief do-not-repeat-this-at-home experience with catheter play and does think it could be quite interesting, with an extreme number of caveats.
First, pup notes that medical caths are usually inserted upon unprepared victims rather roughly, or unintentionally roughly. He is uncertain, and has no firsthand experience, but they may also often be of somewhat excessive diameter as far as comfort is concerned. Most medical caths are "Foley"s and 'lock in' with a balloon; in a hospital bed those are then going to be attached to heavy lengths of tubing that can tug, get tangled or yanked, etc.
A somewhat simpler design is the straight cath intended for temporary drainage by people with various conditions. Pup is aware that a subset of those become skilled at self-insertion by necessity or a desire to stop being manhandled by professionals.
Various healthy people do explore this as what's considered an 'extreme' sort of play. Pup notes anyone embarking on such must first become extremely familiar with the concepts and considerations of 'basic' urethral play (sterilize everything, make every precaution to be able to 'retrieve' what goes in, every surface must be twice as smooth as you think it needs to be, the right-sized 'toy' is always half the diameter you think is reasonable, lube is very important and also must be sterile/antimicrobial) and then the additional considerations of 'going deep' and breaching the sterile environment of the bladder, including the permanent damage that could be done if things do not go 'smoothly.'
At the risk of TMI, pup can say that carefully and slowly self-inserting a narrow, well-lubricated, 'straight' (meaning quite flexible, but with no special locking tip) catheter was a very pain-free and curious experience. The sensation of 'constantly peeing' - more literally, of being held open and freely draining - can certainly be enjoyable, at least for a little while. If the walls of the catheter are thin enough, the warmth of passing fluid can be felt, and of course the reaction of the bladder as well.
Pup thinks that, for anyone dying to experience this, starting with the basics of sounding and slowly working up to brief (hour-in-the-bathroom-type) sessions with a sterile, disposable, straight catheter from a proper medical supply company is "only as risky as it sounds" - on par with getting a piercing, but with different body parts (and potentially one's continence) at risk.
Combining catheters with diapers is a much greater gamble, since something like a Foley would have to be used (more opportunity for things to go wrong; one does not want that balloon to pop), and the 'microclimate' within a warm diaper would be an invitation for infection. Pup thinks he has heard of people accomplishing this anyhow, but only in the context of a few risky hours as a fetish scene, not as overnight or long-term wear. The tip of a Foley is somewhat bulky and awkward - it needs to contain the 'valve' to fill the balloon, be wide enough not to 'fall in,' and provide an attachment point for medical tubing - and pup does not imagine it feeling that great folded into a diaper. Unless you have always wanted to dribble an inch further back between your legs. [Perhaps the situation is different for females? But the risks of infection would be the same.]
Now pup gets into various considerations that the medical establishment (and otherwise) may not be very aware of:
Pup's do-not-try-this-at-home experiment long ago was made comfortable and slightly additionally-antiseptic by use of a petrolatum-based polymyxin ointment as lube. Pup has a suspicion that petrolatum-type lubrication provides more protection from irritation and pain both during insertion and after removal than the water-soluble preparations used in medicine. But pup is also aware that introducing much oil-based compound, or 'external-use-only' antibiotic, into the bladder might not be a good idea. (On the other paw, pup thinks that 'trace amounts' would flush out rather rapidly in a healthy person.) Petrolatum is not compatible with latex, so perhaps that is a reason for medical applications using water-based.
Pup thinks there is huge room for improvement in the design of medical catheters, and potential for the design of devices with recreational cross-purpose. There are many opportunities left to reduce the risk of infection - at the time of insertion it is a matter of sterility and not pushing pathogens 'down the pipe', but with extended wear it is mostly a matter of backflow within and along the exterior of the cath, and how hospitable the materials involved are to pathogens in general. Pup suspects that most current designs are basically hoses without backflow prevention within the tube, and manufacturing in a series of one-way valves would be quite helpful. There is probably opportunity to develop an internal 'locking' mechanism that is more forgiving and less risky than the Foley balloon.
Once technology advances, it seems quite possible to create the ideal 'recreational' catheter, also with medical applications: A locking type, but tipped in a 'reverse' Texas arrangement, meaning a condom tip likely affixed to the wearer with adhesive to preserve a sterile environment. (Pup is biased by being male. Surely an equivalent could be developed for the ladies?) Rather than a bulky external valve and tube for a bed or leg bag, internal backflow prevention combined with a micromechanical or EPAM-type valve could allow the wearer to control their flow at the press of a button - permitting a more normal lifestyle for those with true afflictions, and the possibility for 'extended on' amongst other wearers. ^^
So pup does not fear advances in catheterization, but advances in implant technology that would render such an interim, easily mail-ordered and self-installed device unnecessary!
[Pup also, only now, realizes that his dream fails to account for sexual function, which would become a particular issue if the gadget terminated in an adhesive condom tip meant to stay in place for at least a week. Perhaps pup's idea is not really optimal - which would make the chance of the whole idea getting 'scooped' by an implant more likely - or a co-axial solution might suffice.]
~~
Due diligence shows pup that, in fact, at least one implant already does exist, and 'sling' procedures are becoming a simpler way to help many with real problems. So pup's dream of a switched pipe may just be a pipe dream... unless it would still have a niche as a less-surgical 'retrofit.'