The type approval will be for Cups conforming to the 1990 spec. So I think later 964 Cups will therefore be eligible as long as they are not massively disimilar. That's how the FIA HTPs work for now...
There are 3 problems with this though:
Firstly the FIA HTP (Historic Technical Passport) that you'll need is now a rip-off, needing renewing at large cost every so often and if the car changes owner, it used to be for the car for life.
Secondly, no matter if they allow later cars into the FIA HTP system, events won't necessarily have a class for them. All they do is upset their current clients because newer faster cars are killing them.
Thirdly, the Cups didn't really race in any prestigious events when new other than the Cup series. You aren't going to get an entry to the Le Mans Classic, or the Tour Auto or the Mille Miglia which are the events that add value to a historic car.
So I don't think this is going to be a massive value add, unless Peter Auto add a category for them in the CER or something which might work in their favour slightly.
All IMHO of course...
Rick