I used my PC XT alien probe to check every control lines, data and address bus.
The CPLD was mysteriously keeping address A0 stuck at 0. 

There was no short with ground, so I added an inverter on A0 in the CPLD with the output going to an unused pin. There was no need to make any patch on the PCB (yet).

With the pin A0 configured as an input, it was no longer hanging the bus and I could plug the BBTC on the PC XT motherboard. 

However, the BBTC still didn't work. LDZSYS was not able to connect to the Z8001, and the DIAG program remained stuck.

The bucket simulation seems to work OK.

The listing of the bootstrap initialization program doesn't match the files BOOTODD and BOOTEVEN.