So that worked very easily - thank you @dc42 - for anyone else wanting to do the same - I used this in config.g.
M584 X0.0 Y0.1 Z0.2:1.0:1.1:1.2 E0.3 ; set drive mapping with dummy driver on mainboard (0.2)
M906 X1400 Y1400 Z100 E800 I60 ; set to low power
M584 X0.0 Y0.1 Z1.0:1.1:1.2 E0.3 ; set drive mapping for the real z steppers
M906 X1400 Y1400 Z1000 E800 I60 ; set a sensible power level
M584 X0.0 Y0.1 Z0.2:1.0:1.1:1.2 E0.3 ; add the dummy drive (still at 100mA) back to the group.
This does break automatic bed leveling since RRF3 now thinks I have 4 independent z's and I'm only declaring 3 in bed.g - I don't think there is any easy solution for that but if anyone has a good suggestion...