Yeah. When it works and the cache is intact, it is a marvelous tool to stay longer on the trail.
the 150 mile section south from whitepass to the Columbia river
Using the method I described early in this thread I have successfully cached food near the trailhead on Road 5603, just north of the Mt. Adams wilderness. It is remote, not a busy trailhead at all, and there's enough road damage from the winter of 2015-16 that you'd need to consult a ranger station about which access roads are passable. But the caches I've laid there (twice) worked out fine.
Your next decent caching opportunity, southbound, would be as you exit Mt Adams wilderness at Road 521, or just a bit further on where the PCT crosses Road 23 by the junction with Road 8810. These locations are very accessible compared to Road 5603.
Have fun! It's a very spectacular section through Goat Rocks and Mt Adams, and still pleasant on most of the less scenic stretches.