In theory, if only one buoy was to be placed, it should be a lateral aid (one that marks the channel) rather than an informational aid (like a DANGER marker). I would think it would be red, and placed on the southerly end of the wall - marking the "right" side of the channel. And while we're theorizing, the buoy should probably be a daymark (a channel marker on a pole), rather than a floating Aid.
In practicality, I would guess that the mariners who actually know the "Red Right Returning" rule, are the same guys (and gals) that know enough to obtain Local Knowledge prior to entry . . .. I agree that even with a lateral aid, there will still be a good amount of groundings, cuz too many boaters don't know the rules, and will pass the red buoy (or daymark) on the wrong side.
It's kinda like knowing what the light configuration is on a barge at anchor.
Too many boat owners simply don't know - and we all know that those are the same guys who blame everyone but themselves when they find themselves aground!