The way it's handled in crossposting to LJ is that if the custom security group is named the same thing, then that's where it's posted to; a feature of LJ's interface is that when entries are posted specifying a nonexistent custom security group, the entry is posted effectively privately.

All of this really depends on the eventual G+ API and its limitations.

Edit: I'm saying this rather than "Yeah, it would be excellent to send someone through to the G+ menu if all else failed" because the existing crossposts are handled such that someone can post and then turn off the computer and walk away, and Dreamwidth will keep trying the crosspost on their behalf, and I do not imagine that there would be an operator-must-be-present-through-whole-process thing designed for crossposting to G+.