It has been done, the main concern is placing your finger into the guard activates the "trigger" which makes it fire. At all fields and in all reputeable tournaments a marker must have a trigger, it is defined in the rulebook. Its a neat concept, but will not fly. It is too dangerous, and I honestly enjoy feeling a trigger, and not just putting my fingers in a big slot and wiggling them around.
Google it dude... http://www.automags.org/forums/showthread.php?t=179613
pics are all dead but there you go, let a terrible idea die.