Normally, in this case one would keep one of the engines running until ground crew has produced a serviceable GPU. In the extremely rare case of both the APU and the external power receptacle (on some types, there is more than one of those) not working, it would be the batteries to the rescue. Remember that they are typically certified to power the essential systems for 30 minutes inflight. Maintenance action would typically be required thereafter. And it would be no cause for disembarkation to the starboard side.
On the Fokker 70, there was one little procedure back in the time when I flew it - I do not know if it is still valid. On remote airports with no air starter unit available, with the APU u/s, one was allowed to shut down #1 engine and keep #2 running in order to allow the passengers to disembark. Then #1 would be restarted with bleed air from #2, which would then be shut down. With #1 running, the aircraft would be fuelled and baggage loaded. Then #2 would be restarted (using bleed air from #1), #1 shut down again and the passengers boarded again; #1 started up and one would be good to go.
This is obviously a last-ditch procedure and I have never actually used it. The tail-mounted engines are what makes this possible here as they are way less likely to harm people than if they were mounted on the wing. I have no experience on wing-mounted jet types but would be highly surprised, were there a similar procedure on the 737 or 320.
However, I drifted off on a tangent here. The only situation I would disembark passengers to the starboard side in I can currently think of is when the port door is jammed and there is no other way to get them off the aircraft.
Last edited by Tu.114; 27th Oct 2013 at 15:53.