This can only become as big as Sharkbrew becomes. The clans out there that mass recruit are getting fewer and fewer by the year, right now 6 clans are open and it's all the same people who were clanning 2 years ago with maybe a few dozen new faces.
Additionally, because this system is managed outside of the game, and requires self-reporting, I see it becoming a substantial chore for the Sharkbrew team to manage. It's a little easier to manage Clan ELO rankings when there are only 6 clans, and they all post their aftermath topics to your forums, but the management of the scale between 6 clans and 2 ELO leagues, to 400+ pures split into 1, 13, 25 def brackets, doing Tribrid, and Edge BH style fights is magnitudes of difficulty greater than things that you currently manage.
In addition, if you rely 100% on self reporting by posting results on your forums, the participants of this new ELO league will be few and far between.
Just some of my concerns, not saying you can't make it work, just saying it's a lot to bite off at once, especially without any support from Jagex.