There was a gcode defined to set the axis length many years ago (M208), but according to the Gcode wiki at [reprap.org] Marlin uses M208 for a different function.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].