you couldn't just place 'bigger' chips in there,
the address lines for a start would need to be 1 bit bigger for the extra memory,
what I believe 'should' be possible would be to piggy back the chips, pin for pin, and use the CS line on pin 28 to access the different memory chips,
wouldn't make much difference for games, but a bios hack should not be too difficult to implement
what would be fantastic is if the extra data line is available from the GPU, but not connected!