and there are just no SAS equipped single engine craft out there
The aforementioned BAC Longranger is SAS and AP equipped (3-axis) and certified for SE IFR.
Can a AS350 for instance be authorised for IFR operation? What kind of expensive stuff is required to add to a night VFR one to enable IFR operation?
Appendix B of
this document gives you an idea of what must be demonstrated for instrument certification of small rotorcraft (<3175kg & <9 pax seats).