I can't help with a halfway spot, but I can give you 2 great boondocking sites near Bryce. I have found the Forest Service offices to be very helpful if you will stop in and ask about camping sites. They have great maps too.
Ruby's campground just outside Bryce Canyon has a dumpsite for $5 that has good tasting water.
Barb (RVM18), Sena (capuchin monkey, RVM Head Mascot) & Lily (Maltese/Yorkie)
2011 Shasta Cynara 230F & 2012 Smart Car toad, 272 watt solar system
The Journey is Our Destination. Full-timing since 5/18/2011 and loving it.