I have please - tip:
When You will press NMI and CAPS SHIFT key together so will be returned immediatelly from DivMMC menu routine back to mainboard EPROM /ROM/ and continue at doing NMI ROM routine.
Many EPROMs have special funcions activated by NMI. But with connected DivMMC is not possible to activate it.

---------------------------
ROM NMI is:
0066h PUSH AF (and then is redirected to DivMMC)
0067h .........
---------------------------

So please include to Your
DivMMC ROM:
0067 test CAPS SHIFT key
                       IF pressed THEN jump back to ROM to adresse 0067h
                       ELSE continue in DivMMC
----------------------------

Thank You very much.