Get a couple more hundred active NA players in crpg and you might have a chance at repopulating siege. But I think most people are like me and they want to play with as many people (and the best players) as possible. And you'll only find that on the battle server.
Besides that, the obvious problem that has plagued siege since I started crpg over 2.5 years ago is still the same problem: Unbalanced maps. It seems like on any given map, the defenders or attackers always have a major advantage, this is likely due to the static respawn times, rather than having the respawn times dynamically change due to server population.
Also siege has a stigma behind it, that the people who routinely play there can't hang on the battle server against all classes of players. Strength/plate armor crutchers is the stigma that comes to mind when some people think about siege (who are going to get run down by cavalry, or easily targeted by archers due to their slow speed of movement).
I honestly think you'd fix balance issues as well as get more people in the server, if you changed it to a strategus siege type of game play. Set the timer for 15 or 30 or 45 minutes (I really don't care what), defenders and attackers get a fixed amount of tickets they can spawn, and they have flags to defend like in strategus.