I made a special firmware version for testing this. Basically the way that it works is that it skips all the checks and error messages that would normally prevent you from running a file with the belts fully retracted.
To run a file without the machine moving just retract all and then press play. The z-axis will still move and the cooling fan will run, just the XY motors won’t move, but it will still be doing the calculations.
firmware.bin (1.8 MB)