If you have a U tube made out of CPVC... the best way to get the siphon started with it is to fully submerge it... plug both ends with your fingers while under water... lift it and put it in place... remove your fingers...
But... if the power goes out at 3am... will your siphon break? If so you will be screwed at 4am if the power kicks back on...
Personally... I would never trust an overflow box that needs to be primed before turning back on...
If you can, make a drawing of your concept and post the pic. I'm sure mysel fand others will pick it apart and make suggestions for possible improvements.