I wrote a long answer but my IP Masq box choked while digesting an 250 megabyte tarball of an oracle installation I'm moving from one machine to another.
So here we go, a bit more briefly I'm afraid:
Wizardry
I like this idea, yeah. You could drop a patch into the SDM, but this is probably too extensive for that. I think of SDM patches as being more useful for localized patches that fix a single bug or an a very small feature to the system.
So perhaps you could put the patch file somewhere I could grab it via anonymous FTP? I'm very busy with client work at the moment, but perhaps Dan Wickstrom or one of the other core hackers could look it over.
Subsites
Ouch, yeah, this sounds like a real problem that needs discussing. I have no ideas off the top of my head, perhaps something will develop here ...
Relational Segment Woes
They're better than member-in-a-role because you can express more complex relationships. Whether or not you need to frequently enough to make dealing with any extra complexity worthwhile is an altogether different question, of course!
Dan Wickstrom probably knows more about the parties and groups stuff than anyone else who's currently an OpenACS 4 volunteer...
