Making the previous changes to the instruction also gives a few extra instructions: ST AC->[X,X] and ST AC->[X++,X++].  These are not super useful, so I decided to make a change to give me ST AC->[X++,D] instead.  I can do this by forcing the lower byte to be D when the /OVERRIDE is active.

Current the lower byte MAU select line EL selects X when 0 and D when 1.  I want to reverse this behavior by removing the wired-ROM diodes that are there, and putting in diodes for the instructions not currently selected.  Then I swap the X and D inputs to the lower byte MAU chips.  This results in the same behavior as before, but now EL selects D when 0 and X when 1.

Now I can add one more diode from the EL line to the /OVERRIDE line. So when the /OVERRIDE is low (active), it forces EL to be low as well selecting D.  And BAM I have my ST AC->[X++,D] function for only 3 extra diodes.