74AC645 is what you want to use. Its a 3-state octal transceiver, should fit the bill perfectly. Just tie the Direction with !WR or !RD and !Enable line with !CE signal of the huCard and you're good to go.
Well... That work for ROM HU-CARDS. CD System cards and ROM HU-CARDS with memory mapper won't be writeable that way...
Maybe the pair of 74LS245 could have their dir pins tied to the R/W control line. And an pair of ports of an 74LS32 could be used to select which chip would be active while still keeping the ability of "muting" the chip output if the PCE CPU is accessing the video chip or internal RAM chip.
What makes this mod a bit tricky is the fact that the card slot is connected to the main CPU BUS and we can't "latch up" the bus with an device that cannot be disabled/arbitrated by the CPU (the card is, using it's Chip Enable or /CE pin).
So adding the two OR ports would then "add" the input from a 2 position switch to each one of the buffer chips, allowing us and the PCE CPU to enable one at a time, only when it needs access to the PCE card slot.