Well, I know for a fact that the M5120 I have here on my desk, ROM starts at &H900000 which is the 9Megabyte marker. So yea I cant go past 9MB. 

Unless I add the last MB of RAM to the ROM Expansion address location, but that would require a special extension to tell System 6/7 to use that as RAM. That is something I dont know how to do.