It might be safe but it is very very poor form. In the UK, we have not yet had a GA/jet mid-air, which is pretty amazing considering how many people bust CAS and how many fly below CAS without Mode C, when ATC has to assume that a non-C target "must be" below CAS.
The worst thing would be if the GA aircraft was N-reg
I was thinking about your "few in 500ft, scattered in 1500ft and broken in 3000ft" departure case. There, you might end up circling around, between layers, doing your best to avoid popping into cloud, while waiting for the IFR clearance. It just doesn't seem very practical to me. I bet a German pilot is just going to fly off straight into IMC (assuming non-hazardous conditions of course) on approximately his filed route, which is exactly what people do here in the UK and legally so. But you level off below the base of CAS, in all cases.