Hello Eddy!
Great progress, I was incorrect on the MODE 3, which would be expanded maximum mode ROM disabled. The mode I placed it in is MODE 1, ex. min mode, ROM disabled. The default factory mode is MODE 2 which is ex. min mode, ROM enabled. On this subject, I doubt the maximum mode could be used because PORT 6 (which would be used in max mode as address lines A16-A19) is controlling the select lines of the A/D multiplexers.
The Legend's ECU is located in the passenger's side floorboard. I am very familiar with Q400 - as it's the first thing to get knocked off the board when someones heel goes through the ECU 
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Yes, and if you had replaced the chip while the ECU was still powered on it would have recovered! At the beginning of the program there is a group of subroutine calls that run one time only at reset before the normal program loop starts. I haven't timed it (its at least a few minutes apart) but as the motor is running i think the ECU may periodically reset on its own because these programs are run again.
The stock configuration with the 2-4 line decoder on the Legend ECU is that ROM is active from 8000-BFFF and I/O is active from C000-DFFF (since C01F-DFFF aren't used it doesn't matter). My circuit basically piggybacked this 8000-BFFF output from the decoder with another decoder circuit that was active from 0000-7FFF and then NOR'd them together to get the proper chip select for the 48K ROM. I will work on getting the schematic together as well as testing your method before this week is out.
There is one wildcard here and I've spoke to someone about it once before. There is an area of RAM from F800-F8FF that is referenced in the program. What is very strange is that variables are placed into this memory throughout the program (it's referenced over 100 times) but it is never read from. Also, when trying to scan those memory areas through the serial port I always get the same value for every address (#DB). The person I spoke with said that is is most likely some type of developmental debugging program but I am still not 100% positive with that explanation.
As for the daughter board proposition with reverse-ability, I have you beat! I installed standoffs on the board to be able to change the mode back to stock with jumpers and I installed a secondary socket on top of the original to make changes. I'm not sure if the NSX ECU had these same accommodations for changing the mode, though.
The mode is changed with the jumpers in the upper left corner of the board...
-Matt
