I'm just back from the Library of Virginia. I looked at guardian records to see if the children of Frederick had a guardian appointed and who it might be. No luck there. I did find that Frederick's wife, Mary Goodrich Brigg' father died in 1790/91 and included his daughter in the will. Just passing on slaves after Mary's mother died to three daughters. Nothing about grandchildren. Looked at the two volumes of Boddie's Southside Families and found some stuff to add to the descendants of Samuel Briggs but nothing on Frederick Briggs. Looked a a lot of Brunswick Co. deeds and court records and found nothing more there. So more ruling out than getting new information. Tomorrow I'll try to get to the DAR Library and follow up on the person who submitted the Frederick Briggs letter from prison to the DAR. We'll see.
PS Thanks for the additional information on the Frederick Briggs in Craven County. I'm also interested in those that went to SC. Dolly Briggs came from SC back to Yancey Co and may be the Dolly that was in Brunswick Co, VA.