If the rotorcraft flight manual says you can't fly less than 50 kts IMC, then you can't do it under anybody's rulebook, until the RFM is amended.
There is the rub....."The Rule is the Rule!",
Mind you we have had this argument over and over.
Challenging the Lawyers who write RFM's might be in order in certain cases....don't you think?
After all....in far too many instances...the RFM is written to be a one size fits all recipe that has a sole function of protecting the Factory's legal interests only.