It seems to me to be an act of common decency to mark such anchors as a warning to those who might not be appropriately equipped, and it would be nice if the climbing world could decide on something simple but universally recognized. Surely if sport climbers can hang a red tag on their projects as some sort of "keep off" sign, and trad climbers can mark a dangerous loose block with a chalked "X," the rest of the world could agree on something simple for extra-high anchors. A little loop of red cord perhaps? Maybe a plastic cable tie? A red tag with a decent purpose? Who has a practical idea?
"who might not be appropriately equipped"
How about a little brass plaque at the base of each route stating the length of the pitch, what gear is essential to keep it a "G" rated climb, where the crux move is, and the contact info of the FA party in case you disagree with the plaque.
Sorry for the overly sarcastic response, but this is climbing. We do not need more "signs". As has been said, we need self-sufficency and responsibility.
"Closing the system" is so ridiculously easy it should be as common as making sure your harness is on correctly. That is the simple, effective fix. We do not need to come up with "route markings" to add yet another way to pay less attention to what we are doing.