Im not sure. I thought there was an option on the permissions masks for each forum to be a moderator, but clearly there is no checkbox for that. It is as you said, either you can moderate any forum you have access too, or none.
Having a second login just to moderate a handful of forums seems like a real pain for those who would be doing that. But they would have to chime in on if thats difficult or not (with chrome, and the 'People' menu, its actually very easy to maintain multiple sessions to the same website... its what I do to manage MGS).
I wonder if this 'feature' would exist in the next major rev of IPB. This is running on 3.3.x right now (due to stability with the old pimpbot routines)... but 3.4.x is out. unsure what it has, or what would be involved in upgrading to it.