Without a link to the event I am not sure what you are talking about.  I am sure that they have Porche only events similar to Corvette club which has a couple of their events out of the year that are Vette only.  Since it says all invited I would assume they would be welcoming.  Email the event chair for a clear cut answer.  

A few years ago I raced at a PCA event in Ionia with my RX-7 and the drivers there were very nice and cool guys to race with.  Plus the Porches are a lot of fun to watch run.  I finished 2nd overall.  I doubt they would have been so inviting if I had FTD.

I will be there testing the timing equipment (I hope).  They may not run the regular classes like we do.  In the past, they have had a non-Porsche class.

Curt