If you also have an FAA IR then you can fly IFR as well but only outside CAS
The above is certainly true for G-reg in the UK (FAA PPL with an FAA IR), by virtue of the ANO prohibiting IFR in CAS otherwise, so this is permitted on the basis that everything not prohibited is permitted.
I do wonder to what degree this works
outside the UK. Has anybody looked into this?
I recall reading a UK magazine article 2-3 years ago claiming that this applies in France too, adding that it is not useful because much of France is Class E which is CAS for IFR.
However, did the writer check this out? The CAA decides what privileges an FAA PPL/IR pilot would have in a G-reg, but the DGAC is entitled to make prohibitions in this area too. If this was permitted, then one could do a VFR flight (using one's FAA PPL privileges) and then (having found IFR-only conditions at the destination) he could ask for an IFR clearance (using his FAA IR privilege)
if the destination was OCAS. That sounds like a useful and a legal (though obviously unwise) way of going about things. It could also be a way of getting instrument approach practice for a lot less money than Bournemouth or Cranfield
The DGAC have a whole pile of restrictive practices intended to stop French pilots flying F-reg on the FAA PPL or IR, though I don't think these are in the French AIP. Some of these might apply to G too.