I did it two weeks ago. Award RT IAD-DEN-PDX-DEN-IAD. On the way back I wanted to go DEN-LAX-SIN-KUL, so when I got to DEN (and after the rest of the family boarded on seprate PNRs) I went to the podium and let them know I was not taking the last leg. The GA offloaded me and canceled out the rest of the ITN (last leg, no big deal). My mom-in-law skipped the last segment from IAD to GSP on the same trip. We actually did that at check-in.

The key is, no checked bags because they can't "short check" your bags. (in-law's bag was checked on Mrs Yogi's PNR because Mrs Yogi was supposed to stop at IAD).

In both cases we had additional purchased tickets that we were using to continue traveling, so that probably headed off any lectures.