BaronG,
The reason you cannot quote chapter and verse is that you are simply wrong. Any Part 27 helicopter can be certificated for operation under instrument flight rules (certificated for flight in IMC) by complying with Appendix B to FAR/CS 27. There is no requirement to have two engines; in fact a number of civil singles are already certificated for flight in IMC.
The main reason not many are seen is because there is little or no demand - the cost of the additional requirements for stability, equipment, systems and installation for an aircraft which would rarely use them, is prohibitive.
JAR-OPS 3 prohibits SEIMC for Commercial Air Transport (CAT) as did ICAO Annex 6 (by inference) before the recent amendment (not yet published). SEIMC is now covered by the revised Annex 6 but is unlikely (in my view) to result in a change of regulations world-wide. The amendment calls for additional equipment/procedures mainly required to mitigate the risk of flying in cloud (mostly equipment failure and subsequent loss of control) for the purpose of CAT.
Jim