Is there any way I can get IFR privileges in CAS on G-reg with an FAA IR?
No.
The conversion process (if it can be called that) from an ICAO IR to a CAA issued JAA IR is this:
- sit all the applicable JAA IR exams, as appropriate according to whether it is a PPL or a CPL
- do min 15hrs flying
- pass the IR checkride
Elsewhere in Europe, the 15hrs requirement may be missing (it is not a JAA thing) but you still have to pass the checkride.
There is no shortcut. The full ground school has to be studied and passed, even if you have an ICAO ATPL. That is the vast bulk of the work. The 15hrs of flying is probably not a major issue for most - you can knock that off in a week.