I think it should be doable (though performance might still be an issue for huge documents) — whether there is a real need for it is another question ;)

Anyhow, I'd probably try to leave out Metafont to begin with (IMHO, PostScript has rather superb font handling capabilities on its own). Also, I guess I wouldn't attempt a direct 1:1 reimplementation in PS (except for maybe the TeX parser), but rather a loose mapping of good ideas and features onto the specific functionality that PS has to offer, but with the net effect of generating comparable output. But that's all mere theory at the moment (and probably not even a good one :)

(Actually, I did play with the idea on and off... But then, with life being so short and so many other things being more rewarding in the short run, I never got started (yeah, lame excuse, I know). Actually, I haven't even gotten as far as doing a thorough search on whether other people might already be working on it.  And now, I don't really have a personal need for it any longer — which might keep me focused beyond the fun playing around phase...)