Whilst I agree with Calico and others that it is very unlikely to be put to the test, I have walked through the scenario and my prepared responses many, many times. I had one for situations where people would be aware of the complex demands of my work that might justify wearing. There are times where it is rather tricky to get to the bathroom and I wear and use my nappy with a certain pride when it helps me through those moments, even though it's not technically necessary. For the benefit of others who wouldn't know about that, I rehearsed various semi-truths and blatant falsehoods over the years. These included:
* I'm part of a team testing the fit and security of a new version for the manufacturer. We get paid to do this like a medical trial - it's crazy!
* My friend has a wager on that I can't survive wearing them for a week, it was a drinking game that I lost. I thought they would be awful to wear but actually they are quite comfy.
* I'll be diving deep in a couple of weeks and can't wear a P-valve in my drysuit. So I'm trying to get used to these damn things so I don't have to think about them on the day.
Recently, though, I had a paradigm shift following the realisation that when put on the spot, I had no confidence in my ability to tell a flat lie. So now, for situations where some work-related explanation wouldn't sound plausible, I have prepared myself to come straight out with the truth. "It's a lifestyle thing, quite a few people do it for comfort and relaxation and it really works", i.e. somethng like Jbdl's suggestion. "You should try nappies if ever you get mega-stressed, once you are over the 'weird' factor they are surprisingly comfy and create this aura of calm." I could say that with conviction to pretty much anyone I know, with only a few needing to be kept in the dark about them being for pleasure not medical need. This is all part of a general increase in comfort level with being seen out in a nappy, TBH I don't care who sees or knows these days although no-one has batted an eyelid.