There was a true story around here somewhere where some guys girlfriend tickled the roof of his mouth for about an hour, let me try to find it.
edit- here
http://www.ticklingforum.com/showthread.php?t=169280
I guess other people have thought of the same kind of stuff.
it'd be a weird experience i would say, i've tickled my lips before by accdent and it's annoying as hell.
actually it would be kinda creepy that they have a bondage thing for that though...
When I got my tongue pierced they held it with some weird kind of tongs. I don't know how effective it would be to hold a tongue that was actually trying to escape, but it held mine while I was sitting still pretty well.The Whitehead gag (invented by a 19th century dentist of that name) can be used to hold the mouth open to tickle the roof of the mouth. I don't know of any device to hold the tongue still.