Having 700 rated players in an "amateur" league event is pretty tough. Limit is 730 but that is in the lower ranked pro level, and for BCA which is straight races, it won't be fun.
Seems the top 25% are put in the highest division, but going by what I saw while there, if you are a mid to high 500 you can be in the top 25% easily. Which means B+ players will be playing even with A+ and Open players.
Even a 700 limit would be hard on players that pay their own way there, I guess if you won a trip through your league, then it's a free ride so who cares then really. Just have some fun and see how well you do. If I had to pay my own way to Vegas for a week, no way am I entering an event with players 100 points higher than me. Local Joss event that will cost me under $100, bring it on, spending $1,000 or more on a trip to have someone run 3-4 racks on me while I sit there, nope.
I believe the platinum division was for the top 15%. The cutoff was around 610 Fargo or somewhere in that neighborhood.