With a GAL 16V8 you could block /CS of high RAM (8000-FFFFH),
if you use a 62256 chip.

GAL ASM look like :

CSRAM = A8*A9*A10*A11*A12*A13*A14*A15 + /CS_8032

where /CS_8032 is the decoding line of high RAM whithout I/O
(normally /A15).

When A8..A15 are true (all at 1) CSRAM is inhibited at 1 by the OR.

You could use the product A8*..*A15, inverted, for decoding your
I/O chip.