Sorry, since I started this thread I have acquired both the 100 and backlit Portable ROMs. Now all that's missing is the original non-backlit Portable – and for good measure, the original Portable that has been upgraded (which should be the same ROM).
The idea is to compare all three: Portable 1, Portable BL, PowerBook 100. Apple says the PB 100 ROM is identical to the Portable's, indeed requiring a special CPU GLU logic chip ID register in the 100 to differentiate the two during software installs. However, the PB 100 is capable of SCSI disk mode which Apple says is built into the ROM. I find it unlikely that the original Portable ROM had that much forward thinking built-into it 1989 (especially since the Portable didn't offer it). Also, the original Portable could support a maximum 9MB RAM. The PowerBook 100 and Backlit Portable both share an 8MB limitation. The Backlit Portable was also introduced the same year as the 100, all of which suggests that they share a revised ROM and this is the identical ROM to which Apple refers. If true, then the BL Portable is likely capable of SCSI disk, depending on what else is involved. I expect to find the BL Portable & 100 identical and the original Portable to be different, namely no SCSI disk mode code (Apple also indicates something about video addressing refresh). I am curious about the upgraded Portable though. It required the backlit LCD to be plugged into a special card which occupied the ROM expansion slot. Presumably this provided the extra RAM decoding necessary for the backlight state monitoring, while allowing the original Portable to retain 9MB RAM maximum, or it simply patched the original ROM with the new 8MB ceiling reserving 1MB for the backlight state. Apple's Developer Notes indicate that this ROM card actually contains a revised ROM that replaces the original onboard ROM. I would be curious to see how GetRom would see this patched ROM. Further, since expansion to 9MB was only achievable through third party 8MB RAM cards utilizing the PDS slot, this ROM card may well limit a Portable upgraded with a backlight display to 2MB RAM only accommodated by Apple's 1MB max. RAM expansion slot (which was incompatible with RAM inserted via the PDS slot in any event), or the revised ROM may have allowed it use the 3MB version offered with backlit motherboard (though that was a completely different kind of incompatible RAM with a different connector, unless they made it for both types). It's all very odd really. For instance, I understand the PDS RAM cards were compatible with both models. But if you put the 8MB card in the backlit, did it just ignore the extra MB? And used with the original, upgraded to backlit, did it lose that top MB as well? I also found reference to the fact that the backlight upgrade was offered after the debut of the Portable and abruptly discontinued prior to the 100 intro. It was like a $1500 option. Clearly it was not a very profitable item for Apple which had more economically re-engineered the logicboard to accept the backlit display readily, leaving the RAM, ROM & PDS slots free for future expansion.