The V9990 is accessed only by these few ports (on the MSX):
; Port defines
G9K_VRAM EQU #60 ; R/W
G9K_PALETTE EQU #61 ; R/W
G9K_CMD_DATA EQU #62 ; R/W
G9K_REG_DATA EQU #63 ; R/W
G9K_REG_SELECT EQU #64 ; W
G9K_STATUS EQU #65 ; R
G9K_INT_FLAG EQU #66 ; R/W
G9K_SYS_CTRL EQU #67 ; W
G9K_OUTPUT_CTRL EQU #6F ; R/W
So you only need a range of 16 port addresses and can fully use the V9990, which has its own video ram. So maybe it is pretty easy to build an interface for it. SymbOS supports a resolution of up to 1024x212 at 16 colours.
But I agree that a Graphics9000 output wouldn't look like an Enterprise anymore, but it would be a nice project anyway
On the MSX it was a little bit different, as it is still looking similiar to the VDP9938 etc. Though even here there was not that much software released for the Graphics9000, a huge part of the apps which run with the G9K are now coming from SymbOS.