Due to the attribution scheme I used, that would be really complicated. (But thanks - your reply was much appreciated, I'm likely to use it at some point.) My workaround, to be scripted, is to iterate this: 1) select by attributes (over a list of stored expressions), 2) make layer from selected features, 3) export the layer to a feature class, 4) select all from the feature class, and 5) dissolve.