It'd be way cool for it to appear to be hanging there with no supports, Boomer. You could put threaded dowels, bolts really, thru the case. If you're not going to have it open, then you could reinforce inside the case with plywood to hold the bolts/washers/nuts.
These bolts would be positioned on your stud spacings, centered on the studs of course. Then drill slightly oversized holes through the plaster/drywall into the studs and epoxy the bolts into those holes with a quick set epoxy.
Voila, the floating violin case shelf.