Frost would never have entered Clan Battle Royale if it wasn't for the clan board customization. Some of us are still going in-game from time to time since the event, even though we lost pretty much immediately.
There is something to this idea, but when all is said and done, I think the in-game clans will dominate the clan rankings. They simply focus on that aspect on the game and are thus guaranteed to be better at wars. I believe by giving out forum-related prizes you're more likely to get in-game clans into the forum, than forum clans back into the game with this idea. This doesn't necessarily have to be a bad thing, however.