Looking through an installation manual of the Garmin GTX 3X5 the flight ID can definitely be changed and there is some aircraft specific information such as category, length, width and speed; although, it doesn't allow for a specific values for any of these. Instead you choose <= 15 meters, <= 150 knots, etc. A Cessna 172 and a Kitfox are pretty similar in size, maybe enough that these values would be correct for both aircraft. You can also select unknown. From what I can tell only the Flight ID is the only value that is specific enough to really differentiate between a 172 and a Kitfox as far as ADS-B goes, but I'm probably wrong somewhere. I will see what kind of answer I get from an avionics shop, but I'm guessing no matter the feasibility the answer will be "no".