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Dieter,
I think one of the problems with trapping Syrphids emerging from rot holes is that the larvae frequently leave the hole and pupate elsewhere. However, some will find somewhere dry at the top of the rot hole and so will remain as a pupa within the rothole. However many do leave as larvae, which means that you will not find them by covering the rothole with an emergence trap for adults. I have bred Myathropa florea in plastic bottles at home and most of the larvae crawled up the sides of bottle into netting placed over the bottle. These needed to be removed and kept in a new container, with sterile but not completely dry substrate in which they can pupate.
The best technique is certainly to remove some of the rothole material and breed out at home/in the laboratory. However I have only ever managed to breed Myathropa florea in this way, nothing else yet.:(

Good luck with your survey.