I have tried:
* p_Mega_TK - the most successful so far, but somewhat of a trouble to accurately convert my old materials to look right.
* ctrl_buffer - Haven't gotten it to work with Max, but I think I could. But, it doesn't really seem to do what I want as far as a straightforward way to output different material properties (maybe I am misunderstanding it, though.)
* Max built-in functions - This always render the scene over again for each element (which is OK for quick tests, but longer renders at 5x render time would be very long). But when I made the final composite, it seemed to be missing some data - the edges didn't really match up, and in general it didn't match the combined output image.
So far, it seems like the Mega_TK shader is the best - I will just have to get used to writing my materials for that system.
So, finally the question: given that it seems to be a standard practice, what configuration are people here actually using to render separate elements? Thanks for the help.