I agree with IO540 and I fky high and use quadrantals when in IMC in Class G 'cos that's what you are supposed to do and it's designed to arrange separation without radio aids at all.
I accept the issue with having to cope with narrow corridors in the Southeast but I also agree that it's usually a reasonably safe assumption that non transponding traffic is actually below the cloud base. I base this on considerable experience of having a RIS above the cloud and never seeing any reported non transponding traffic!