I think I'm with Shark actually, that calling both mages alive a draw and be done with it.
Basing it on remaining Life is the fairest way of splitting that tie with any ease, but it's still artificial, and doesn't actually represent the gamestate in many cases. If the game ends with with me on 1 Life but with Initiative on the next turn and a Flameblast in my book, whilst you have 2 Life, then there's no realistic prospect of your surviving the next QC phase, but you get the modified win when you start calculating levels of draw (and any other system would have a similar problem).
A lot of metagame considerations disappear if you consider not killing the enemy mage to be a win. It should be a very low scored result (eg: 4/1/0), and I'm not even averse to calling it a loss...
Of course, if you have the luxury of untimed rounds, then that's obviously best. A midway solution is to untimed all but the last match, and give them 15 mins after the penultimate match completes. It's not perfect, but it removes the chance of a stray 4hr match, whilst still making most untimed.