Its surprisingly easy and foolproof - locking all-in-ones.
Previously I ran into a place that sold nice footed jammies that had a locking zipper on them, I will look and see if I can find that again. The option was discretely shown. Ahh here it is, babypants:
http://www.baby-pants.com/jammies.php
Or you could go the formal route:
http://www.theoriginalinstitute.com/LockingClothing.htm
https://buy.special-need-products.com/Back-Zipper-Pajamas_c_118.html
https://buy.special-need-products.com/Bodysuit--Barrier-Garments_c_131.html
There's a niche market specifically for blocking diaper access discretely. For daytime/work wear, you could just get white ones and trim back the arms and legs to turn it into a bit of a onesie like some wear during the day, looks like you're just wearing a tshirt.
Either way, if you can't get your all-in-one off, you're not getting at the tapes
And it's much easier to secure the zipper on that than it is to try to protect just the diaper.
Though... if you get one of those all-in-ones with the zipper on the BACK, that may be all that's necessary, depending on the wearer's flexibility.
And one final idea I had awhile ago... if you punch a hole and install a grommet at the top material beside where the zipper stops, you can simply use a wire tie to tie the zipper pull to the grommet. Trim the tie flush with a little dikes and you have a perfectly trapped toddler for awhile as long as you hide/lock the cutters away.
That would probably work fairly well in the front, and should be pretty foolproof, cheap, and universal for the back. This could work to modify a lot of different things - you could even do this to prevent removal of pants. I remember seeing awhile ago someone that wore zip-up button-fly jeans. He had problems with them unzipping and popping off, so he zipped up, turned the pull upward, tied it around the shaft of the button, then buttoned the fly. The zipper wasn't going anywhere, and it was 100% discrete. Again you're not getting out of those jeans without a nippers.
I have a fleece all-in-one from these guys, they are expensive but build to suit, and offer a full length back zipper option:
http://www.the-all-in-one-company.co.uk/