Ken,
I have found what could be very helpful information in finding the PCA Island No. 10 flag. A newspaper article Times Picayune, June 20, 1906 says the Bushnell's (daughter and grandson of Union Major John Vincent Johnston) allowed the Richmond Confederate Museum to keep the flag "for safekeeping until it could be finally placed in the proposed Battle Abbey."
The Battle Abbey is part of the Virginia Historical Society and I have e-mailed them asking about this flag. They have not gotten back to me yet.
Jack