Quick answer, I''m fairly certain that 14 CFR Part 135 (carrying revenue passengers) prohibits Single Engine IFR with non-turbine powered aircraft. I don't have the exact rule handy, but Part 135 speaks to a requirement for an approved engine monitoring program. Oil analysis and compression checks don't suffice... 135 requires a higher level of safety when carrying passengers under IFR.

135 Cargo only is OK for SE piston under IFR...