Maybe there’s a ‘graceful’ way in Mel but I don’t know of it. Hopefully someone else does.

Your best bet is a simple plugin command. You’d give a recursive function a face, it looks up the edges of that face, then the contiguous faces of those edges, then the edges of those faces etc. at each step you’d need to add your new components into a container like an std::set, if the components were already in the container then you stop that particular instance of your function. Once your last instance is complete your set will contain all the faces in a particular shell.

The problem is that MEL doesn’t have efficient containers like std::set, you’d have to store your faces in a array. This algorithm has an exponential time complexity so if your mesh contained more than a few douzen or hundred faces your MEL script would grind to a halt and freeze maya.

But I’ve been away from maya so I might be wrong, there might be an easy way.