I agree, tweaking valour and other such aspects to fit siege is a good idea.
The current issue I find myself having when thinking about siege is that avoiding combat is often one of the best strategies, so how do you reward good team play for actually defending/going after the flag and not just TDM? I don't think multi by itself is enough, really.
Most siege maps are clearly designed to have progressive choke points (ladder/ramp, then gate house, then flag, for instance), so you can't punish defenders if they effectively hold the first choke point. An score offset bonus for fighting within a certain radius of the flag seems like it would be do-able, and would at least reward people for fighting there.
I personally don't care about the actual rewards in siege (I'l probably never gain another level), I just want the server to be populated because I enjoy playing it. I feel like the main things that kill NA2 are:
1. Less XP/Gold accrual rate than Battle or DTV
2. Bad maps that clear the server
3. High repairs and no multi when <8 players so no one wants to help populate the server when it's empty
Fix at least 2 of those problems, and NA2 will come back from the dead.