ok I have un****ed the phone. I couldn't get RageAgainstTheCage to run (never got a root shell), but the newer KillingInTheNameOf worked.
In /data I found that the lag fix had renamed data, system, app, app-private, dalvik-cache with .old suffixes. The ext2 volume was there but nothing was mounting it on boot. The directories had been replaced with files just containing the text "stump".
For now I nuked the files, renamed the .old directories back to what they should've been, and the phone is working again. (Still no lagfix though.)