Interesting idea. Do other programs use this sort of thing?

Using environment variables for "simple" configuration of "base libraries" is pretty standard in the Unix world, right? Using JSON not so much (probably because most of them predate JSON) but that sounds better than introducing one new variable per rcparam.

Actually, a better idea would be to either have MPLRC contain the contents of the rcparams file itself, or have it point to a filename that contains the desired rcparams (in which case process substitution can come in handy) -- thus avoiding the need to invent yet another encoding.