No one redesigned that exactly - if someone did, please contact me iam interested in a working design with additional RAM.

If you are just looking for nonvolatile memory, you can get 1MB ROM + Max 512KB FRAM with no additional components aside from an inverter and some coupling caps.

Connect ROM CE and OE to /CART0, and the highest address bit to AUDIN (if 1MB, otherwise not needed).

FRAM CE is on /CART1, OE is connected to AUDIN, WR is connected to AUDIN through an inverter.

 

It's very nice and simple for layout.

 

But if you are looking for more scratchpad memory, the cartridge is not practical at all.

Source: Worth of "released" protos