While it would help to have no upkeep with less than 8 players... I personally don't find it to be much of an issue. If you've got nothing to lose (or gain), why not just use peasant gear? Obviously you are only playing on a sub-8 player server since you are a die-hard siege fan, and you shouldn't really be having too many financial issues if that is the case. Could be wrong, but seemingly everyone who is willing to play with less than 8 players doesn't really need gold all that badly (high gen, long time player, etc.). Or if you do need the gold... peasant gear. Multis with only 4 people would be nice for sure since you can usually get at least 4 people to go at once. It would be really frustrating to be defenders on a 2v2 though.
My suggestion would be to create a "NA Siege Map Balancer" type of position. Yes, we've heard it all before... when they went looking for scene managers, no NA stepped up, so we only have 2 EU players managing the maps, etc etc. We get it... but is it really impossible to open up a small position where someone can simply veto siege maps from the NA map rotation, or slightly modify EU maps before they become part of the NA rotation? This person wouldn't necessarily need to even make maps from scratch, although surely NA-tailored maps would be great. Not sure how the servers are set up... but I would imagine that it would be possible for NA to have different maps than EU, although for one reason or another that has yet to happen.