Certain websites I've been to have had an RPG battle system built into them, others I had to make rules of combat from whole cloth. However the combatants fight, the real intrigue of the Battle Royale lies in the roleplay and strategic elements. Some players will put all their points into the Battle Ability, and hope that superior EVs wins the game for them. Some will really on the Intelligence Ability, hoping that Choice Items, Life Orb, Leftovers and etc. will prevail over a team with basic items. Some will use the Stealth Ability, and scout out which teams they can safely handle in a fight. But some players won't rely on abilities at all, and will instead use roleplay to set themselves up with a strong group, backstab the weakest member, then pass the blame onto another member.
On forums where there was an average of 50 users on at a time, I've found interest in Battle Royale, and that was without Shoddy Battle (which I believe will only make BR more awesome). With 366 people on right now (and 800 earlier today), I'm sure we can fill up the roster.