You can use the following inside a custom macro to set the rapid speed as a percentage. You can pass a value to the macro from the gcode and set it. e.g. M103 V2000. To set it as mm/min you would just have to calculate the percentage of the max rapid speed.
I've just thrown this together... Not tested..
function m103(hParam)
local inst = mc.mcGetInstance();
local maxVel = 3000
if (hParam ~= nil) then
local vVal = mc.mcCntlGetLocalVar(inst, hParam, mc.SV_V)
local vFlag = mc.mcCntlGetLocalVarFlag(inst, hParam, mc.SV_V)
local RROpercent = (vVal/maxVel)*100
if(vFlag == 1) then
mc.mcCntlSetRRO(inst, RROpercent)
end
end
end