You don't need startup clearance for hotel mode - provided that airport operator is consent with using no2 with prop brake while the aeroplane is being serviced. Also don't turn on the beacon - it means that startup is imminent and usually causes the ground staff to run away. One rated pilot has to be seated in the cockpit all the time the engine is running in hotel mode - if master warning goes off, CL2 goes to FSO before even asking "What was that?"
We used powerback a lot and I often see green-tailed Lufthansa ATRs power-backing at MUC.