Having stuffed mushrooms a few times, I find that it's best not to be overly ambitious with stuffing. As in, you need very little stuffing to properly stuff a mushroom, so keep it simple. The crab idea sounds amazing, and I'm sure lobster would do as a fine stand-in. I usually just mince the stems and cook them in butter with shallots, then deglaze the pan with something nice like port or sherry (sherry and mushrooms are made for each other). Mix the cooked mushrooms with seasoned bread crumbs, stuff, and broil until golden.

Crab yes, and "themselves" would otherwise be known as duxelles which you can prepare using the stems.

Crab! or themselves, butter, herb & bread crumb.