I think the likelihood of needing a vastly different rule set other than "don't double-move/be a jerk" is very low. Most conflict will be within your own neighborhood, regardless of the number of civs in the game. Any conflagration larger than a regional skirmish will hopefully sort itself out if everyone makes a decent effort.
Edit: would it be possible to mod in a "lurker" faction with an invisible kill unit? This would be a unit with unlimited (or virtually so) number of movement points and attacks per turn, and an abusrdly high attack strength so it never loses, etc so we could wipe a civ off the board if its presence became disruptive? This would be the equivalent of a mod kill and the civ would not interact with the map in any other way and would avoid having to retire to AI. This civ would also be able to have unlimited (or again, virtually so) espionage points so that lurkers would have a good overview of the game. Just a thought.
Edit: would it be possible to mod in a "lurker" faction with an invisible kill unit? This would be a unit with unlimited (or virtually so) number of movement points and attacks per turn, and an abusrdly high attack strength so it never loses, etc so we could wipe a civ off the board if its presence became disruptive? This would be the equivalent of a mod kill and the civ would not interact with the map in any other way and would avoid having to retire to AI. This civ would also be able to have unlimited (or again, virtually so) espionage points so that lurkers would have a good overview of the game. Just a thought.