Wow, I do not have the time and patience for that. Considering the last one would not even take bait for a week which was not even in a trap. Anyway Sunday I will try physical removal of rocks.Mantis shrimp are just as spectacular as the pistols. In hunting, they utilise cavatation to increase the damage of their hunting strikes. Glad to hear that you will try to find a home if extracted. the more you look into mantis shrim, the more unbelievible complex they apear to be.
Good luck freeing the tank of the shrimp. Perhaps you could use very fine fishing line placed in the sand as a noose so you could noose it when it comes near. Works for me when I trap worms from my tank. Here is a link to my trap set up (post number 131): http://www.thereefuge.com.au/threads/dr_schells-chriss-5ft-x-2-5ft-x-2-5ft-mixed-tank.87/page-7
Watch this space.