I guess it must be a combination of me overcompensating for the meter and underrating film degradation. One was ORWO 27, some 30 years old, which I rated at ISO 160, but probably that was too optimistic. The other was Ilford XP2, which was just past its expiration date, so I shot it at its nominal ISO 400. I guess I need to try again with a more reliable camera.
I also have a 100ft spool of Konica VX 400, some 25 years old - I shot one roll, and it came completely black, or rather dark grey (including the perforated sides). The lab swear they didn't mess it up, so it must be an extreme case of degradation.
Thanks for the tip - an angle viewfinder probably does make better sense than getting another camera just for the purpose. Is the Olympus one better than others, or just any would do (there seem to be similar finders from Pentax, Mamiya and Yashica around)?