by setting a nighttime which is bigger than only the 8 hours, you would automaticly choose primetime for yourself. If f.e. 14 hours nighttime, there are 10 hours left which you could call primetime. This is more flexible than a server defined primetime of say only 5 hours, as there are still time differentials between f.e. factions based mainly on players out of russia and those out of england. So to leave it to the leaders of a faction to choose a sutiable nighttime and therefor primetime i believe would be better than an overall fixed/setted server primetime.
I see also a problem with the small battles, they are needed to get rid of spies, unwanted traders or just for fun ^^, having them on a separated server could make sence for factions, even if those battles would take place at the same time. You could choose people who would be in the one or the other depending on priority.
Everything else i could think of would limit the number of battles and may be seen as overall restrictive. F.e. Having only battles with numbers greater 200 on attacker sides or attacker needs to be at least over "lightly armed" or within 10 hours at least a certain number of people need to have applied for either site and be shown in roster(that but is again problem as the battle times may change constantly) or players with 100 and/or no faction couldn't be attacked at all(pro trader contra spies) or ....