Yes, the ROM is here:http://byuu.org/temp/st0018-20120225.tar.bz2
It is 0xf3.bin.
From the SNES side, the register ports are:
$3800r data port from ST018 to SNES
$3802r data port from SNES to ST018
$3804w command register (results in $3800)
01 = unknown
aa = board upload
b2 = unknown
b3 = player lost test
b4 = player won test
b5 = check test
f1/f2 = test registers (returning $00 seems enough)
f3 = dump ROM
f4 = dump RAM(?)
$3804r status register
d7 = chip ready
d4 = write ready
d0 = read ready
... and that's it. Very small interface.
So for instance, you want to know if player 1 was checked.
You write $aa to $3804. Then you write the board into $3802 (9x9 grid + 16 bytes for promotion information.) Now write $b5 to $3804, wait a bit, read $3800. It will be 1 if checked, 0 if not checked.
I can jump on IRC/AIM if someone wants to try hooking it up to an ARM core in MESS. Right now, I'm not even sure where to begin execution when the chip powers on.