As it turns out I wasn't quite out of the woods....

The aftermarket alt I put in (as I said, it came off a '94 850 donor) died about 3 days later.

I decided I'd had enough fooling around, so I took the day off work and took the original OEM alt ( call it ALT-A) and the aftermarket one (ALT-B) to a starter and alt repair place to have them fully tested and find out what the hell was going on.

ALT-A had been pretty much ruined due to oil ingestion from this incident: http://www.brickboard.com/AWD/volvo/1332188/V70/oil_leak_2_help_cam_seal.html

Oil is apparently very bad for your alt and regulator.

The guy said he could rebuild the rotor, but he would need to order the regulator from Montreal and it would take ~5 days to get it. Cost ~$180 + time waiting for parts.

ALT-B had burned its regulator out. (The regulators on the two alts were different, BTW, so I guess you can't always 'mix and match') The guy told me the original regulator was of very poor quality, and he had in stock a replacement part that was made by Bosch. He could rebuild that one for me that afternoon, and the rest of the unit appeared to be in pretty good shape, though not of as good quality as the OEM Bosch unit. Cost ~$85.

I had him replace the regulator in ALT-B and it has been working fine since then.