Yep...just realised I've been stupid. Theres no need to make a "userport" addon to expose more lines for ROM selection as I can just add a new hardware register(s) that do anything I want. idiot. This is in fact what Solder did on his 1Mb ROM expansion.
I wonder if you could add RAM instead of ROM and just write to it and page in in/out.... I know that writes fall through to the RAM under it, but if it could write to both, than while you'd lose 32k of RAM under the ROM (or rather pretend ROM as its now RAM), but gain megabytes of parged RAM for doing whatever you like....I wonder.. I suspect someone with a bit more skill would have to try that.
I think the 1st thing I'll do as an add-on, is a new register that sets some LED's, this is the basis of everything you add on a computer really, so it should be pretty good fun.
(Oh...and whoever designed the "8bit baby" has never plugged one of them into a Plus/4 in their life!)