Fernie - I used to have to sellotape ds1 into his nappy - and since I'm right about everything, you must be being utterly reasonable to do it too!
Seriously, ds1 used to take his nappy off if he did a poo once he was in bed, and I'd find him later, fast asleep, covered in dried poo, with poo on his pjs (or worse, his bedding) because he'd just whip the nappy off, dump it, and go back to bed. It was far harder getting him clean when it was dried on, and meant waking him up too.
BTW - we weren't being deliberately neglectful - he seemed to be pooing as soon as he was put to bed, and we weren't spotting it until later. We did try telling him he should call down to us, and we'd come and change him straight away - so the next night, he did call us - and we found him in his bedroom, stark naked apart from a thick layer of sudocrem, and a layer of poo on his bottom too.
There was more poo and sudocrem on the couch and in the carpet - most of a big pot had been used - and whilst dh scraped it out of the soft furnishings, I showered ds1 3 times to get it all off him.
After that, we resorted to sellotape.