This was fairly easy.
First, insert this patch
into the ROM.
Next, patch the following code with Asar.
if read1($00FFD5) == $23
!addr = $6000
!addr = $0000
autoclean JSL Main : NOP #$02
LDA $76 : BNE .return
STZ.w $13F3|!addr : STZ.w $1891|!addr
That should be all you need, and it should work as you want!
Note that we found it quite difficult to create this code with UberASM, so it became an Asar patch.
If you need to use UberASM, please let me know. I'll do my best.