I have spent some hours designing a printed circuit board for this design. This will be a board that plugs in on top of the Pepino board. I did this on purpose using only through-hole components just in case there are people interested in building one of these. Personally I prefer SMD components, and that would have allowed the board to be somewhat smaller.
I also revised the schematics, to be able to support the entire TMS99105 feature set. That required some more signals. Since I'm already using all the pins available on the Pepino board, I put in a couple more buffers and a latch to store control signals. That will meant that the already multiplexed address/data bus will have another phase (under FPGA control) to read the additional CPU output signals (such as BST1, BST2 and BST3) and to latch control signals to the latch.