I have had customers wanting to drive from the floating end, and while it is not ideal, it will most likely be ok.
All stepper motors have a wavy washer or two behind the back bearing so this can take up at least 0.5mm.
The coupling will also help.
On another note the thermal expansion would not be that much unless you are really working the ballscrew and it starts to heat up, a Z axis is not that dynamic with most moves happening over a small area, and not up and down over the whole stroke.