It's a sample size of 1 (2 if you include Mrs. CJ) but I checked bags on a very similar itinerary: ORD-LAX-ICN-BKK-USM, in C/F. Bags arrived in better shape than we did. Plus, the BKK-USM was a separately purchased itinerary, which UA had no problem adding to the interline.

I say, go for it!