A quick update:
The bad news is that there was a fundamental mistake in my MLDL design. The good news is that I fully understand it and allows me to do a redesign of the state machine which will make the MLDL less sensible for metastability and easier to update future enhancements.
For those who are interested: I could not get the WROM instruction to work properly. It seemed to work sometimes, depending on the program that I used for initiating the WROM. After hooking up a logic state analyzer to the prototype the fault was quickly determined. I assumed that the HP41 cycle first transmitted the contents of the C-register, and then the instruction to be executed. In case of the WROM instruction, the C-register contains address and data for the WROM to operate on. This assumption was wrong, the C-register is sent AFTER the instruction.
I am currently redesigning the state machine and will hopefully be able to rebuild and test the prototype during the weekend.
Meindert