I decided to have a peek at Prohibition and it appears to be writing to the CRTC at 0x3d0 and 0x3d1 to do the "scrolling" (writes to regs C and D to set the start address).
I tried compiling with the crtc ports mirrored at 0x3d0/0x3d1 (VGA_SetupOther) and the game does seem to work. I don't know how safe it is to mirror these ports, perhaps for CGA, TANDY and PCJR but not VGA?
http://www.seasip.info/AmstradXT/ppctech/section1.html
Might not be present on real cga systems??
Aha... according to this list:
http://bochs.sourceforge.net/techspec/PORTS.LST
There are indeed mirrored ports at 03D0!
Though
http://www.seasip.info/AmstradXT/ppctech/section1.html
is way more explicit about WHY they are duplicated, but well.
Oops, didn't read all the way through that one. 😊
But, at least we know that it happens on non-Amstrad systems as well.
Think the machine=cga is fine for that, the game seems to be playable with it.