It doesn't make sense for Squall and everyone else to know about the Griever GF either. His conversation with Rinoa makes no sense if that were true. Instead of explaining that the ring is based on the most powerful GF, a huge winged 'lion', instead he just says he likes lions and doesn't mention anything else.
Well, he calls his WINGED LION icon 'Griever', and he also considers Griever the most powerful GF. We know these things as facts, not assumptions. It all in the game.
I'm sorry, this is IMO one of the best indications of R=U there is, and I'm yet to hear a better possibility.
How, exactly does this support the forbidden theory? If R did = U, then she wouldn't waste time trying to 'frighten Squall with his ring', as you put it. There are times in the game when it's mentioned that Rinoa's powers will be passed on and eventually reach Ultimecia, who lives in the distant future. There's the fact that R and U have nothing in common - speech, motives, appearance, actions - which suggests very little connection. Besides, the game's entire romance aspect is rendered completely devoid of hope and meaning if Squall's just going to end up murdering his his girlfriend, who at some point is going to become a muderous witch.

My theory about memories being carried in Sorceresss powers and passed on is kind of similar to R=U theory, but without the 'direct connection' between the two.