The Influences list in the Paint Skin Weights tool is a
treeView. You can get a list of existing treeViews like this:
There were two names in my output, and for me, the one we want has the id "theSkinClusterInflList".
I don't see a way to query what's selected in it in the treeView documentation (nor wrap the selection callbacks, as I can't query them either), but I do see that you can get the children, and you can ask specifically if a child is selected, so we can roll our own get-selected:
def getPaintWeightsWindowSelectedInfls (tv):
kids = cmds.treeView(tv, query=True, children=True)
return filter(lambda x: cmds.treeView(tv, query=True, itemSelected=x), kids)
Now I can get the selected children like this:
Hopefully that answers the question. Feel free to clarify further if not, or ask any follow-up questions.