Thanks Phil for the interesting question.
There are many things you could do in these situations. First of all, STV automatically sets the tag property of auto generated cells to their corresponding class definition property index. Second, I always find it useful in these situations to set the tags of different detail models in the tableViewModel:detailViewWillAppearForRowAtIndexPath:withDetailTableViewModel: delegate method. You can set the tag according to the specific cell they correspond to, or according to their depth level (e.g. root model tag=0, 1st level tag=1, etc.) You can then always access the cell's model using its "ownerTableViewModel" property.
Please tell me if this is enough for your case.