After some uninteresting attempts with Arduino, I found Mecrisp-Stellaris Forth (http://mecrisp.sourceforge.net/) which works great.

This lead me to wonder if there are any 9P implementations in Forth (surely not?)

Yes! there is one by iru (https://github.com/iru-/9p4).

Then I thought about the impossible: a 9p file server in mecrisp forth. I discovered someone had already done exactly that: https://hub.darcs.net/pointfree/ForthFS

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