Not a big deal. The TSA, DHS or other security people don’t care that you are wearing diapers any more than they would care if female flyers are using feminine hygiene products. The failed Underwear Bomber last year did obviously raise concerns but for the most part, the pat down process probably won’t detect anything unless you are wearing the bulkiest diaper you can find. In which case, you are just looking to get caught.
I flew from and then back into Toronto in the days leading up to the G20 conference in June. On the flight out, on the monitors the security warning was that if you had any concerns (medical, religious, etc….) about the pat down, to have a word with a security official and it could be done in a private room. I was going to but decided against it. Flying back (two days before the G20), we were warned that we would be searched twice – once upon boarding the flight to Toronto and then again when we arrived in Toronto . I went through the normal screening at the airport I was flying out from but was not checked at all once we arrived in Toronto .
I haven't gone through the full-body scanners, so I can't comment on that, but I suspect that if the screeners haven't seen anything to raise alarms, they don't care what type of undergarment you choose.