Just a quick question -- in Assault on Osgiliath, the Ancient Harbor and The Old Bridge locations state "the players cannot travel here."

This refers to the actual act of traveling in the travel phase, right? "Cannot travel" is absolute, but my understanding is that "traveling" and "becoming the active location" and separate and distinct.

Would it stlil be legal to make that location the active location by alternate means (West Road Traveler, Thror's Map)?

You are correct. You could still make those locations active via a card effect, you just cannot travel to them during the travel phase.