Yeah, I'm also using spot rendering quite often, mainly to fix details that paint.net (I hate photoshop) can't fix.I haven't tried this myself, but have you tried using the spot render to do sections like that?
But for "full renders", such as these, I just tend to do a batch that runs at night while I sleep and then I merge them all. Or, as I sometimes discover in the morning, the batch is unusable because I was too greedy and tried to force too many characters on the same renders and Daz crashed in the night...