Snail or hermit? For the hermit I don't knw of a way that wouldn't potentially hurt the clam other than above...For snail...hopefully it would't struggle like a hermit..just go into it's shell...thus the clam would not try to stay closed and the snail could escape..provided that gravity woudl get the snail out after the clam opens up..cuase if the snail has to crawl it's way out the clam will close again and you'll be back where you started. Or maybe you could snatch it after the clam opens, but before the snail makes a run for it and gets trapped again.
Cheers,
grimmjohn