I don't think the microtype package could feasibly be made to work with plain, since it's necessarily tied to the LaTeX font setup, in my understanding.
I'm not looking for a general solution. I basically just want cmr to be microtype-ized to reduce editing of TUGboat articles purely to deal with overfull boxes. (The nicer output is a pleasant side effect.)
XeTeX's lack of support for font expansion makes it uninteresting to me. (As far as I know, there isn't any feasible way to support font expansion with DVI output, but if someone is up for that major development effort, great. :).
Maybe it's been done and I don't know about it? Maybe it's simple and I'm being too scared off by the loads of definitions necessary?