I asked diptera.info (which tends to attract European entomologists) and got a response "The parasites are almost certainly a Monodontomerus species but I have no idea what the disc could be."

If this is a controlled release of M. dentipes we would have an explanation for the possibly synthetic "nest".

Thanks for the IDs. A British page on the family notes:

"Species of the non-British tribe Podagrionini (Toryminae) are parasitoids of the eggs of Mantidae. Sometimes females are phoretic on the adult female mantid, thus ensuring that they have access to freshly deposited eggs which they must reach before the froth of the ootheca hardens."

The USDA page says "A large number of species (all Podagrionini) attack mantid eggs."

Eric Eaton thought the white mass might be a mantid ootheca.

The Podagrionini that I'm familiar with have the hind femur swollen, much like in the chalcidids. I thought the white mass looked more like some sort of suet block.

The white stuff is still on the rock, months later. I can scrape it off if anybody wants it or wants to know what it looks like inside.