[kbs666]

I'd start with either system neutral or other and customize as needed. Other as you point out is still pretty fantasy oriented so you might have less "waste" in a system neutral realm.

As to modeling the campaign, the top level structure should be the various factions. below that would be solar systems or planets, depending on how detailed you want to get.

You might find uses for the adventure area topic. A battletech RPG may not have classic dungeons but there will be times when the party has to explore a particular area in or out of their mechs and the adventure area is the topic meant to model that sort of thing in RW.

For military organization, just like everything else, add what you need.