Well if it's against the rules somebody is gonna do it
Trouble is that people would have kept it fairly quiet so might be difficult to find out how prevalent it was.
Although slightly different circumstances -on 69 sqn in Malta - Warby and co took an experienced camera fitter with them whilst flying operations in Beaufighter
PR a/c -these brave volunteers changed magazines/fixed snags etc and acted as lookouts for enemy fighters.
I do not have a book to hand but a quick google comes up with 3 names...
Corporal Liebert, LAC Norman Shirley , LAC Ron Hadden. Although I have not read the book for years I believe that at least 1 of these men was decorated with a flying medal (DFM ?) -which in itself was extremely rare for groundcrew.