20.3 A boat moving astern by backing a sail shall keep clear of one that is not.
The key phrase there is "by backing a sail." If you can back up without backing a sail, as can be done with a lot of boats, you still have rights over the boats behind you, because they are still considered overtaking boats.
Am I wrong? I can back up my boat forever with the sail just loose and weathervaning.
Yes.
The "overtaking boat keeps clear rule" was binned ages ago
Once there is an overlap "normal rules apply" - however, there needs to be room and oppertunity to keep clear so how long you would beed to be given to slow down, stop and then reverse could make an interesting discussion - you do not have to predict a Right of way boats action, only avoid it - so the "over the line, slam the breaks on and reverse back" method would be (IMO) dodgy to say the least.