I have no knowledge of such a conveyor, doesn't mean that they don't exist.
I believe there is such an adapter which was used with the .55" Boyes rifle. HWC made them, so the Boyes could be fired on a .22" range.
This is a common enough system used in civilian convertors. They are awkward to use. I would think that from a service standpoint, having a little insert would be a nightmare, the things would get lost, and the conveyor would become useless.
BadgerDog - We could do up a genuine .22" "tanker" conversion. If it worked for Navy Arms, why not? You could prepare a photo spread illustrating all the unique features. Actually, I used some odds and ends of salvaged parts, sleeved a No.4 barrel to .22, and made up a rifle not unlike a UK No.9. Lots of messed up Lee Enfields with which to tinker. We have all seen the photos of mudpuppy's remarkable Delisle reproductions.