Well I did some searching and found this
thread discussing the use of these
Brey-Krause brackets. But it sounds as if these brackets are only for GT3 race seats, not stock seats. I've sent an email to Brey-Krause to confirm that, I'll let you know what they say.
In my googling I also came across these
extenders claiming to add 3 to 6 inches of leg room on the driver's side in a 986 or 996. Perhaps they would add that kind of legroom in a 996, but I don't see it in my 986, maybe an inch if they work. But then the seatback is also going to be very upright and really pressed against the rear bulkhead.
Well for me, it's not a necessity. I've lived with the car as it is for 13 years. If there is something easy to bolt right in and retain my seat slider, that would add even a half inch of extra headroom for $189, I'd certainly buy it.