TG/PCE internal BRAM mod hu chip replacement

Previous thread appears to have been lost in the recent unpleasantness. If I'm mistaken please merge

My goal is to replicate the custom chip that acts as a mapper/controller for the BRAM in TG/PC expansion systems.
Based on pictures of the of a Tennokoe PCB I know how the A/D/etc lines are connected between the system, chip and ram. Based on the BRAM tool source code I know how the PCE communicates with the chip.

What I'm not clear on is how to interface the chip to "ports" $1803/7. Is it simply memory mapped? The IRQ line isn't connected on the Tennokoe PCB.

Also, in follow up to the guy who pointed out the problem plugging an expansion device in with this mod might cause I've found that the Tennokoe makes it self known using the CDD line so a soft switch to disable the mod when an external device is detected would be easy.