These three commands

mov a,096h
mov 096h,a
mov 096h,#0

All get the error message:

***** NON-EXISTENT INTERNAL RAM ADDRESS *****

whereas, replacing the 096h with an 0a0h (or other, more commonly used address), works fine.

Looking at the website, they don't claim support for the 8252 chip, but I'll send them an email on it anyway.
As for the possible errors, that makes sense - Just to be on the safe side, I think I'll do two obj files, identical except for one with 0a0, the other with 0b0, and replace only the ones that match in both files. Thanks!