You won't find anything in JAA-FCL because the IMC Rating is a UK National Rating. An IR can count as an initial test for or revalidation test for an IMC Rating. The only advantage in doing this is that the IMC Rating is valid for 25 months rather than the IRs 12 months - but you'd need to have the IMC Rating entered into your licence. If you have a UK CPL (or ATPL) then you have similar privileges to the holder of an IMC Rating and these are valid as long as your licence is valid. However, a JAA CPL/ATPL does not carry the same privileges and is a strictly VMC licence unless you have an IR.