HP-41 MCODE, unused bits in chip 0

[hth]

I have some need for at least one unused bit in chip 0 (the status registers).

I am thinking of using the 2 unused key assignment bits that belong to the second half of the ENTER key. These are the MSB in nibble 7 of register 10 and 15. It seems the ZENROM uses these bits.

I have two questions:

1. Are there other modules apart from ZENROM that use these bits?
2. How popular is ZENROM these days, do you use it?
   

Håkan