Let's ignore the current UU tier for a second.
There are currently 53 pokemon in BL. Just eyeballing that list, nothing stands out as being obviously better than everything else in BL. Everyone keeps mentioning Rhyperior as almost a sure-fire broken pokemon. But, I see a lot of stuff in BL that could give Rhyperior trouble. Perhaps the rest of you don't agree with me -- but Rhyperior doesn't strike me as "OMG Amazing!" compared to the rest of the BL's. Yes, it's a great pokemon, but if it was head-and-shoulders better than the other BL's -- it would be OU. And it's not. It's BL.
My point is this -- the current BL list has a bunch of solid pokemon in it. They obviously aren't as good as OU pokemon, that's why they are BL. But, they are quite powerful. And there are a lot of them - 53, in fact. If you just took the current BL's and tossed in a few top UU pokemon (Steelix, Claydol, Venusaur, etc) -- that might be a playable metagame right there. We'll have no idea until we try. I don't know how you could possibly sort it out in a couple of weeks.
I think people are way too hung up on the name "UU".
Imagine this -- what if we created a BL Ladder right now? Only Borderline pokemon are allowed. How long would it take to figure out which BL's were completely dominant and which BL's were severely outclassed? Two weeks? I don't think so. It's possible it would never happen. Because if all the current BL's were relegated to their own metagame, it's very possible that no pokemon would stand out as "way too good" or "completely crappy". They all might be "good or bad, depending on the team". You could argue that if 53 pokemon fit that description, you have a balanced metagame right there.
Basically, the "BL Ladder" is what is proposed here. The difference is that this new ladder will not be exclusive to BL pokemon. Anything below BL will also be allowed. How many below-BL pokemon (intentionally avoiding the term "UU") will be playable with all these powerful BL pokemon? I don't know. It's possible that UU pokes like Persian are completely irrelevant in a metagame with Rhyperior running around. But then again, pokemon like Tangrowth and Flygon may have a field day, and keep Rhyperior in check. Who knows? I don't. But, I think it will take a lot longer than two weeks for people to figure it out.