Sleeping in a bed at Rock Creek Lodge would be nice, but a 2 hour drive to the Portal early Tuesday morning would kind of suck.
I agree it would suck to leave that Rock Creek Lodge bed at 1am and drive a couple hours to get on the trail by 3am. I think your current plan is the best - stick with the camp reservation at the Portal for rest and convenience. As for other possible alternatives, have you considered camping or acclimation hiking at Horseshoe Meadows/Cottonwood trailhead ~10,000 ft. It's higher elevation than the Portal (if you really want that) with a short drive to the trailhead. This would be preferable to Onion Valley IMHO, but you might get the best rest staying at the Portal. Some people don't sleep so well at 10,000.
