Sorry BEagle, don't see your problem. There is nothing to prevent an existing IMC rating holder taking that rating with him to a JAR licence and continuing to maintain it exactly as he does now. If the JAA licence holder mainatins a single-crew IR then the IMC rating is automatically revalidated with the IR, and lasts for 25 months. Considering that the JAA CPL is, and always has been, a purely VFR licence (akin to the UK BCPL), that seems entirely sensible.
If, however, the CPL (or ATP) holder has only a multi-crew IR then the IMC rating must be revalidated every 25 months by proficiency check. Again an eminently sensible course of action.