Do they specialize in rove beetles?

I can't speak to the genus, but I have these notes on Proctotrupidae: "mainly endoparasites of beetle larvae, solitary or gregarious... generally those inhabiting leaf litter such as Carabidae, Staphylinidae, and Elateridae... Phaenoserphus viator lays eggs in Pterostichus ground beetle larva; when larva approaches maximum size, the 30 or so larvae feed rapidly, destroy host, and then each makes its own hole in the integument of the host’s ventral surface, and crawls out for about ¾ of its length, leaving only its posterior end anchored; they pupate without forming a cocoon, and all of the pupae are oriented with their heads oriented toward’s the host larva’s anterior end." An illustration of this in Askew closely resembles the scene you've captured here.
All of this seems like a perfect match for these pupae, and it seems to me that these images would be a good addition to the (almost non-existent!) section for that family.

I just stumbled on Taro's blog here, which has a lot of similar photos.

I wonder why Taro didn't steer me to his work. Fascinating stuff.

These pupae all died. Too much handling maybe.