I knew that querying specific nodes of the xml from UPS was out of the scope of eCart's support.
Since I have that part done I thought maybe it would now come down to a special use of the eCart Shipping rules. However I've been unsuccessful setting the Trigger and Calculations of a 3rd rule here so I think you are right, I need to go back to the xpath query for some tweaking.
I have been studying the count() function of xpath. I will see if I make any progress. If not, I will do as you suggest and sign up for that ticket.
Either way, I will report back with my solution in case others come across this.