I've only been to the Gunks about 50 or so times. So there are still a number of areas that are still quite unfamiliar even at the cliff base - Ok, actually some that I have never even seen, forget familiarity. There are also a few places on the carriage road where I am quite sure where I am.
This addresses only the idea of marking the carriage road - not all of the sarcastic responses about marking the cliff, nor the good suggestions about asking cliff-side about routes from someone who is familiar.
The mileage marker idea might be useful to bikers and hikers. It would be useful to climbers as well, but only after they have marked up which ever version of the guide book they have, to correlate the distances with the climbs. Some of the guide book trail information is now inconsistent with the existing trail structure.
Marking the carriage road with the climb name or at least the area, however, does not depend over the long term on the locations of the trails. So, for example, the sign that says Arrow always tells you where you are on the carriage road relative to the climb. Likewise McCarthy wall could be correlated to any number of past or future guidebooks. The marking need not correspond with trail locations to be useful, only the cliff.
I have always thought that it would be nice to have some small permanent signage. If I want to just hike and enjoy nature, I can do it without spending hours to drive there. If I want to climb and enjoy the company of the people I am with, or happen to meet, having the signs would free me from keeping track of approximate hiking times or looking for landmarks that I might either not recognize or simply walk by. I have even missed the Minty Tree and I know what that looks like.
Obviously my regular partners are not some of the people who always know where they are. I have only been climbing a short time. I often bring people up who have climbed even less. But I also have often visited with people who nearly live there and even they sometime "get lost" on the carriage road.
Signage would be nice.