The main thing keeping me from trying to code this is that I'm not atm sure
how I'd implement the shell.
My idea is a slightly Bourne-ish shell for ProDOS, implemented in two
pieces, which uses the D000-DFFF area where the Selector normally sits to
hold resident state (D000-D2FF, as normally, holding code to run when a
program exits; but instead of this being a Selector it would reload the
shell and copy its state back down (this would include stuff like a search
path for binaries). There's prolly a special filetype I could use for
commands where I could special-implement whatever argc/argv encoding scheme
cc65 apps expect (and I don't know this atm).
I got a bunch of sources of unixy utilities under BSD-ish licenses I could
mash onto the system as well, at least in theory.