The only way to correct for variable probe trigger height when using G29 is to edit the heightmap.csv file that G29 produces and then run G29 S1 to load the modified version.
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].