Flower, I think I can expand on your comment. If you are talking about N864, then part of that airway is delegated to Cardiff ATC. Anyone other than CDF has to avoid non-squawkers because they may be on a procedural crossing with Cardiff. In the MoU there will be a comment such as Cardiff will try to inform other users of this situation, which is obviously not practical. This is why you may often get calls from other units asking if you have any procedural crossers. If you say no then they can legitimately deem that non-squawker as being below CAS.
We at Cardiff have to apply the same rules to Airways traffic as everyone else , traffic must be wearing both Mode A and Mode C.
The only time we once had a non squawker ( I can't quite remember why but it was some emergency I believe ) we had to inform all possible users of the airspace ie those who have autonomous use of the Airway, so it isn't infact an issue at all.