I've been in touch with Eric who is coordinating an 8 hour enduro at Grattan on 5/30, 5/31. He asked me to post about it in case some folks here would be interested. It is NOT and SCCA event. All that said, Grattan is an amazing track to drive and this sounds like a neat way to get lots of seat time there.
Check it out here: grattan8hour.com. Reach out to him (email@example.com) if you're interested and/or have questions.