BTW, you can stick/tack on full FAA (checkride with FSDO inspector or DPE) INSTRUMENT HELICOPTER on validation of foreign licence (would be PPL privileges in US, if need more info, search 61.75 etc). If in US doing IR(H) and having experience to pass FAA private or comm, why not do that as well? Simpler for future, as piggyback licenses can be pain for any changes (new verification letter to FAA, which lasts 6 months etc).
IMC rating, as UK specific, had aim to facilitate flying in tosh UK weather, improve safety etc, from how I see it. Since it's mainly for dealing with IMC conditions and flying S&L plus standard rate turns, climbing/descending through some overcast etc, in AEROPLANE suitably equipped, I can't see it happening the regulators rewrite the regs to suit couple helicopter pilots.
Remember, the EIR only exists due to pressures/lobby of groups/UK CAA/individuals to morph IMCR to EIR in EASA system. Was there heli specific IMCR? No. Thus no reason to transfer any existing rights/privileges to EASA.
No, I'd rather be in f/w in IMC than R22/44, btw. I'm FAA IFR rated (uncurrent) and I'd rather not be in unstabilised Robinson or small heli in IMC, but if current and not in canyon, wouldn't be bothered by bit of milk outside the cockpit.