Yes, my Sabine Pass book has maps of both forts. The site of Fort Manhasset was rediscovered by W.T. Block, who has an article about the fort on his website at http://www.wtblock.com/wtblockjr/fort1.htm
Fort Griffin site is closed right now due to storm damage. The site of one of the redoubts of Fort Manhasset is very close to the route that goes to Sea Rim state park but is on private property.