Ok, I can confirm that this (in the input channel gcode):
; Set dummy filament diameter and then load filament settings M200 D1.75 M703 ; report filament diameter? M200
Does get me into volumetric extrusion mode, and that the M703 does result in a change in the filament diameter. Somewhat bogus way to have to do things, but this does solve my problem for me.
Thanks for the discussion, it helped.