It Will Exist...?
Grindstone Loop Trail is a great idea... and has been for decades! Unfortunately, it has gone mostly unrealized. The Topo Map makes it appear to be there, connecting simply with Grindstone Trail (91) and the Grindstone Spur Trail (91B). Unfortunately, neither of those trails really exist up near the loop.
Logging activity sliced and diced the top of the Mesa, eliminating the other routes in that area. The original Loop Trail was never developed.
Now, there is action on developing the primary Grindstone Trail coming up from Mine Canyon... and a replotting and development of the Grindstone Mesa Loop Trail. The original loop, as shown on the map, fails to approach near the perimeter of the ridgeline. The new survey markers are expanded out to touch on the very edge, at points, then curve back into the dense tree cover to enjoy a variety of what the area has to offer. It should be nice, once completed.
For now... you are hiking off trail. You cannot really get lost... simply hike to one of the edges and follow it around to the inevitable roadway. Follow the road around and down off the Mesa.
Hopefully, they will do something about rehabilitating the roadbed areas. They are a major distraction to the area.
Back in 1998 then again in 2002 the guy who wrote the guidebook for the hiking trails of the Lincoln National Forest talked optimistically about these trails soon being done.
Here we are over a decade later... should we get our hopes up?
WARNING! Hiking and outdoor related sports can be dangerous. Be responsible and prepare for the trip. Study the area you are entering and plan accordingly. Dress for the current and unexpected weather changes. Take plenty of water. Never go alone. Make an itinerary with your plan(s), route(s), destination(s) and expected return time. Give your itinerary to trusted family and/or friends.