Technically legal – but not advisable and not really within the spirit of wild camping.
Others will follow suit and then we will be scraping human jobbies of our bike tyres. You might be a responsible camper but other people will not. Then trees will be chopped and then GT will restrict access.
Yer asking for trouble from neds with cars if you hang about the car park. A forestry commission car park near civilisation is NOT a place to be at night. Rural polis do check these places and may wonder what you are doing.
If you really must, away up to the top somewhere out of sight.