Do I correctly understand that you're interested in writing (also) a PLATO terminal for the ’64 or ’128?
I'm curious if a limited-capability terminal on a ’64 would really be usable for playing most of the games on PLATO … ? but seems like you're definitely more the expert than I …
In particular: remapping from ( 512 × 512 px = 64 × 32 ch ) to ( 320 × 200 px = 40 × 25 ch ) or ( 640 × 200 px = 80 × 25 ch ) in a non-disruptive way.
It looks like, per your text file, the A][ version centered a 192×256px screen and expects its own special 4×6px custom character sets? That would certainly work “passably” on a VIC-II, as long as the 4×6 fonts exist — and with some weird color artefacts across 8×8px cell boundaries, of course.
On the other hand, it seems like the VDC on the ’128 should handle 512×512 @ 16×8 just fine — definitely as 640×512, with margins blanked on each row, although it wouldn't surprise me if some fudging with the VDC registers wouldn't allow for directly mapping exactly 512×512, albeit with not-precisely-square pixels.
I've played around a bit with (including submitting the odd patch to) Linux PTerm, I'd be interested in lending a hand if possible.