When racked, is the power button on the back of the Mac Studio reachable to power on and off the macbine?

It will depend on how you use your set up. You can remove the front panel to access and then place it back,  but with the faceplate in place it is not possible.

We would suggest looking into using a sliding rack shelf such as R1498-SL-3UK, which would allow the Mac to still use the faceplate, but with easier accessiblity to turn it on and off.