Filling up rosters is never a problem if you've got a big battle at a reasonable hour. People come out of the woodwork and ask for slots. As has been said, people are quite often turned away from battles.
This on the NA side of things.
As for incentivizing battles: Fucking silly. How about, instead of making the whole battle process inconvenient, we make initiating battles rewarding outside of the battles themselves, on the Strat map? If you just want to get people to fight battles, then eliminate the Strategus map. Apparently, Strat's not fun enough to justify it's existence outside of the whole battle thing.
If you want battles, deal with the things that are really keeping battles from happening. I'm thinking of massive stacks of troops and gear in everybody's castles, and the almost paradoxically difficult process of recruiting enough troops as a small clan. I'm thinking, too, of the pre-WWI style, massive alliances that occasionally pop up and prompt diplomatic stasis. I'm thinking about the unpredictable geography, such that you cannot maneuver for a tactical advantage in the field based off of terrain--why fight a field battle at all, if it's just going to be a steady grind for both sides? Or, if there's a chance that the Terrain Gods will fuck you over.