I have created fonts for my "fictional scripts"[1] using MF[2] and "converted" them to TTF by importing the bitmap font into the background layer in Fontforge and tracing it with potrace. It was a long time ago, I used little metaness beyond setting weight through pen/path width and/or a general slant trough `currenttransform := identity slanted ...`[3] and/or modifying heights, and the conversion method may be less sophisticated (though it avoided needing to remove overlaps!) but still! I have often thought about doing a more ambitious project using more metaness and maybe MP. I have experimented a bit playing with `superellipse`, `superhalf` and my own `super_iii_q` (super-three-quarters), directions[4] and defining different serifs or lack of them through macros but it never got anywhere.
IIRC Johan Winge used MF for his Tengwar Annatar font for Tolkien's Elvish script, but AFAIK he has never revealed which conversion method he used.
[1]: As in "scripts used for conlangs used in fiction". The languages do exist in a more or less rudimentary form and the scripts definitely exist. I have used some of them to write English or Swedish on occasion.
[2]: I have cerebral palsy so I definitely am not god with GUIs. Unfortunately I also have mild dyscalculia which makes math hard, but not as hard as GUIs!
[3]: This seems not to work in MP. I wonder why.
[4]: I defined N, NNW, NW, WNW, W, WSW, SW, SSW, S, SSE ... as synonyms of up, down etc. and intermediate points between each other, so not very sophisticated! Also top_lft, top_rt, bot_lft, bot_rt, set to different values to get/not get Fraktur-like counters.