I believe these are the same individuals I photographed yesterday, but now on the path, not on the tree trunk. See 4431182. Third photo shows the trees, fourth shows them on the path with trees in background. @donhe, @RogerF, @ibaird.
Sorry Roger, it was a couple of days ago that I took the above photos, and they were on the move, in fact they were on the path, and I nearly trod on them myself, so possibly other walkers did, or if not they would have moved from that location. Though someone like @AlisonMilton (who knows exactly where they were) may have been able to rear them, I am afraid it is probably too late now.
If you do happen to see these distinctive late stage larvae again here's a couple of references- Froggatt's notes https://www.biodiversitylibrary.org/page/3348559
Some general rearing info https://bugtracks.wordpress.com/rearing/
I've also been advised Vermiculite can be successfully used as a pupation medium instead of soil.