Even if SE could do this, I have to wonder how they'd actually determine your ilevel. I mean, a search of players could turn up an ilevel 117 player, but that player could be wielding an Eminent weapon while wearing level 30 gear for all you know. I just don't think there's a viable way for SE to do this.
This thread is about the equipment search function on this site not ingame. They have an average ilvl function in XIV which takes an average off all the slots and if a slots hasnt got anything in it by player choice that slot is at 0 lowering your ilvl. If you are on a job that cant equip anything in the sub weapon slot then that slot isnt counted.
Theres no point having a system like this on XI as theres gear thats lower level thats still useful which will lower your average ilvl.