Pilots have been self-separating in IMC in Australia since before Dick twiddled his first knob.
They do it by using a mix of Company procedures, maths and common sense.
If they were supposed to keep to ATC Sep Standards, they would need to complete an ATC course...or....you provide ATC to maintain the standards.
The success of their efforts can be seen by reviewing the number of collisions between IFR aircraft in uncontrolled airspace....it seems to work.