Very very difficult problem!
In my opinion, there are no pure privateer teams in GT500 class.
It's because they get great financial support from 3 manufacturers more or less.
For example, all Nissan GT500 drivers have a contract with NISMO.
NISMO send them to all Nissan GT500 teams.
Team Bandoh (#19) calls itself privateer.
However, according to Juichi Wakisaka, they get financial support equal to other Lexus teams since 2014.
So I think all GT500 teams are at least semi-works team.