I have been folowing Your Disassembly/Analysis thread for a long time. I dont own a supra, but i own RX7 (fd3s). It has denso ecu, and inside there is a D151803 controller. I have bought chiped ecu. It has external eprom added, but data is scrambled. It looks like this: http://page.freett.com/na6/FD_ECU02_480x360.jpg
I saw that you wrote, that you had made simple board to run external eprom? Maybe you could share it? My car runs rich on idle/low load. I would like to adjust VE table.