I was able to get a hitch from Whitney Portal back to Horseshoe Meadows without any trouble at all. In fact, the couple that picked me up drove me the whole way. I waited all of 10 minutes at the WP trailhead before I got my ride.
This was on September 8th, a Saturday. You may find mixed success if you try to do this on a day other than Saturday, I would imagine it especially challenging mid-week.