Okay sounds like a plan. I mean, some planets will be easy to see the cap right? Say I go to Hoth and there are two instances. The instance I am in currently has 80 people....I switch over to instance 2 at this point and see 45. I'd think it would be a safe assumption that the level cap is around 80-90 if there are no other instances. Not certain but I'm sure some assumptions can be made through this trial and error process.
Sure you won't get the exact number per planet but at the very least perhaps it will shed some light on which planets can hold the most, thus illustrating what you are trying to accomplish. How many planets are there to choose from where PvP is even accessible? Sounds like finding this information out, at peak hours, won't be too hard to find.
Nar Shaddaa is really the first planet where PvP is possible. its not really until you reach 30 before you can go to the areas where you will see the other side.
Tatooine is kept pretty separate though accessible, except for the Dune Sea where PvP is rather common and you are sent on quests to areas where the opposite side frequents (Czerka dig site is a shared dungeon where pvp is quite common).
Alderaan starts out separate, but the questing zones intermingle and you frequently see other players from the opposite side.
Quesh Its split down the middle
Hoth Intermingled questing areas
Belsavis Start separate (but accessible)
Voss a shared outpost inhabited by the Voss (meaning the speeder pad and quick travel points can be used by either side).
Corellia Starting areas completely separate. Main questing areas are kept separate
Ilum Shared PvP Area . Questing areas separate but accessible by either side.
Information concerning planets I found online and parsed it for easier reading. It's not new information but information for reference.
I know that I've seen 70 or so in Section X without a second instance but I still feel like the cap is around 80-90 an instance for later planets. That may be faction cap only....that's another metric to find out lol.
Hopefully a spot will be found but after last weeks turnout, it's hard to imagine any planet holding that many players at once with the current caps
EDIT: Check your inbox on this site, I don't want to burn out any eyes with my walls of texts.