FWIW, I recently spoke to UA about this topic-

They were running a YVR-SYD promotion of around CA$980 through SFO, where I happen to live. Comprable flights SFO-SYD were about US$1500, all in.

So my plan was to take a one-way to YVR, fly the YVR-SFO-SYD-SFO legs of the itenerary, then jump ship in SFO on the way back. The CSR said that it would not be a problem as far as the mileage goes; I would be credited for any legs that I actually flew, and there would be no risk to my Mileage+ account...

I thought the "Hidden City / Point-beyond" rules apply more to the first leg; i.e. my ticket would be invalid if I didn't fly the first YVR-SFO segment.

Unfortunately, the SFO stop is considered a transit since it is continuing on international, so the 'point of entry' customs clearance wouldn't happen, and my bags would continue on to Canada. A subsequent CSR wasn't amused about my claim that if I didn't board the SFO-YVR flight, my bags shouldn't be on the plane under the homeland security 'no-person-no-bag-on-flight' plan... and therefore should be in SFO instead. Probably in some customs/security lockup point anyway.

Anyway, just my recent experience-
Cheers,
Rattie