Hi Kris,
Highly unlikely that they are parasites of the snails.  You said they
are "maggot-like", and almost certainly that's just what they are,
common fly maggots.  It doesn't take flies long to find somethind
dead and lay their eggs on it.  Some years ago a couple of friends
and I did some collecting in a dumpster in back of a scallop shucking
plant in Maine.  We got some nice shells, but it wasn't the most
pleasant collecting I have ever done.  In addition to the smell,
there were GALLONS of maggots.
Paul M.
Rhode Island