As a previous poster said - it might be legal but would you want to do it?
Some of us are required to do it to get the job done. Thankfully, to fly IFR, aircraft must be transponder equipped. TCAS is a wonderful piece of equipment. Coupled with what ATC service can be obtained, it's deemed an acceptable level of risk.
I'd rather fly IMC in Class G than mix it below low cloud with any number of non transponding aircraft flown by pilots who aren't looking out properly or talking to ATC because they're pre-occupied with basic poling and navigation. I.e. Most weekends!