Many thanks for document, very nice.
My approach is little different, for easy soldering (the MCU with many IO is not easy to soldering), my idea is use ATMEGA324 or 644 in TQFP @ 16Mhz and 6 MCP23017 SOIC (16Bit IO Expander I2C) = 96 Key (i needed 95). The Joystick and mouse i use IO from MCU.
But the idea of Greenious is not bad, i have some keyboard from 520 and 1040. I have read on this forum the HD6301 used it’s the same of TT keyboard. Or other posibilty after dump firmware on keyboard, use HD6301 with external ROM but i have not found the schematic of MEGA/TT keyboard on web. This solution as great advantage no programming needed.
TT030 4/32 TOS 3.06, Mega ST4, Mega ST2, 1040 STE, 2 x 1040STFM, 2 x 1040STF, MegaFile 30, Lynx 1