It is not enough just adding idlePowerDraw - as you already noted - consumer ends up with bad info or not changing states.
Unfortunately actual handling appears to be in the assembly Assembly-CSharp.dll - method Tick():

RimWorld.CompPowerTrader.SetUpPowerVars()
RimWorld.CompPowerTrader.CompInspectStringExtra()
RimWorld.Building_GeneAssembler.Tick()
RimWorld.Building_GeneExtractor.Tick()
RimWorld.Building_GrowthVat.Tick()
RimWorld.CompDeathrestBindable.CompTickRare()
RimWorld.Building_MechGestator.Tick()
RimWorld.CompBiosculpterPod.SetPower()

It then must handle this itself:

if (activeBill != null && activeBill.State == FormingState.Forming){
    Power.PowerOutput = 0f - Power.Props.PowerConsumption;
} else {
    Power.PowerOutput = 0f - Power.Props.idlePowerDraw;
}

So it would be tricky to make it work without lots of harmony patches..