It's very much a YMMV scenario. In this particular case, the routing rules forbid it; however, an agent might look at that itinerary and deem it reasonable -- DTW isn't that far out of the way, so it doesn't look like the OP is trying to game the system. If a couple of agents say no, there's not really much OP can do; the SDC policy officially limits you to valid routings, and I sincerely doubt that TPE-SFO-DTW-ORD-PAH is a valid routing on a UA TPE-PAH fare.

This is where the discussion we had recently about nonstop vs direct comes up. If there were a direct (same flight number) flight from DTW-PAH, whether it had an enroute stop or not, the app might offer it.