I think the following should have a chance to reward valor in siege:
For attackers:
-Spending the most time within the flag capture radius
-Most kills within the flag capture radius
-Doing the most damage to doors in a round
-Deploying the most effective ladders (hard to quantify, but maybe someone can think of a way)
For defenders:
-Spending the most time within the flag capture radius
-Most kills within the flag capture radius
-Being the last man standing on the flag to end the round (after attackers have already started to lower the flag): usually this guy is the true hero of the round
Don't make it based on overall kills though. I've had nearly 20 kills in a siege round before without contributing significantly to attacking or defending the flag, usually by killing random archers on the wall. I'm not proud of this behavior, so please don't further reward it.