Honestly, I don't see much difference between buying brain and buying pocker/roullete chips.

You use both of them to play and get back then back if you win. Later on, you exchange them back to money/service.

On BK:
memberhips = money
brains = money

So membership is money valued service which and since you use brains to purchase something brains can be considered as money
and then you can go to another discussion of gambling.

So I don't think using brains as an entry fee completely eliminates possibility for gambling opportunities.
It depends on country and lawyers opinion. In addition, good lawyer can convince the jury ...
So why not just have is as before: give membership as a prize?
Comments are more then welcome :)