Life Orb boosts all damaging moves, and will deduct HP after a successful attack hits, and so in your case, after however many turns it takes doom desire to hit.
I assume that you know the general trivia, like the boost, and recoil, and such, so I won't elaborate...
Source: Smogon
It's just how I interpreted it, I might be wrong...
This was confirmed in the actual Pokemon game by the user Flafpert. So it was a flaw in Showdown that it didn't take Life Orb recoil.
