I think some native siege maps had a system where attackers got X number of lives and defenders only got one, or something like that - not sure if the maps were more defensively biased than crpg ones (which are made to allow many plausible routes to the flag).
Native doesn't need to be balanced, it doesn't matter if attackers win more of the time than defenders, that's why the lives thing worked. The native (and Napoleonic Wars) siege servers that use a lives system generally also have much longer round times than crpg siege. In native with 30 minute round times, if the attackers win in the last, say, 5 minutes, it's pretty much a win for the defenders. Crpg has the dreaded multiplier, so it just wouldn't work here.