I'm pretty sure this was answered long time ago and not being allowed, however the hoops you gotta go through to prove it is just not worth it. Just have a healer stand back and keep an eye on it. Not like we get any points for healing at the front line. Offense or Defense, I get the same amount of badges.
When you only have a few players in Siege, that's true -- but once you get above 10 I think, they start breaking them out into the different categories.
When I first started doing Siege as a level 30 or so, the only thing I could do was farm the towers, and there were a couple of times I was put into the "Support" category because we had more than (whatever the number was) of offensive players in.