It took a few hours yesterday, but I thought of this belt path to be able to move the roller across the Y axis and spin the roller whilst having the motors mounted elsewhere. I was able to think this up because, a few days ago, I got to print on an Ender6 and get to stare at the CoreXY kinematics in detail. The should help with keeping the x length of the roller assemby as close to the build volume's x length as possible. 

The only issue right now is that I didn't expect the assembly to be 300mm instead of 260mm. Part of this is likely to do with the use of a 57mm roller instead of the original 50mm to get 179mm of rolling area instead of 159mm.

It is expected that the roller gear will be 48T, the pulleys 16T and ball bearings 604zz, with a belt of height 1.38mm and non-tooth height of 0.63mm. The pulley flange is 18mm.

I wonder if I can use sensorless homing for this axis or if I'd have to also accommodate a limit switch.