These have to be saved and restored by the ISR
Right, like I said you'd have to call the SavePBP/RestorePBP routines in ReEnterPBP-18.bas/ReEnterPBP-18LP.bas like DT-INTS does to save the PBP system variables.

The rest of what DT-INTS does (and some of SavePBP) would just be repeating what the hardware has already done.