Sorry to be a pain but can you give some top down shots of the spare assembly with Z axis close to in place has possible.

Also of the Z axis like pic 1538 in post #11 but tuther way round.!

Don't think this would be a major job to mount in traditional way but without clear pics of clearances etc then it's difficult to tell.
Thou less than the ideal I'm sure with the width of the Y axis carriage that some form of offset screw could be adapted if needed and eliminate any ugly screw sticking out the bottom and risk of damage to screw.