No - it's not a wind up.
I know of two occasions when it's been done. One is the flying organisation of which dsf speaks (You're obviously not Will or his old man, so who are you dsf
)
The other was the C172 pilot who was killed at EGHA when the seat back collapsed. It was requested that his ashes were spread in the same way which they managed by using a tube positioned outside the aircraft and a drawstring to open the bottom at the required moment.
Keef - don't know what the rules are but in addition to the above we also had an ATCO's ashes spread on a military airfield I once worked at (he'd had the coronary in work). Judging by that I guess they may not be as strict as you suspect.
CM