So my suggestion would be if it’s possible to change the elo system the way that u gain +elo of the leaving player devided by the players who are still in your team in game if u win or u loose that amount less if u stay and loose.
For example:
Dota starts with 5 on 5 the Teams would win/loose 10 elo. (but it could also be implemented in other elo games)
Now 1 player of team 1 leaves the game 10/4->2.5 so if Team 1 still wins they gain 12.5 elo or if they loose each one should loose 7.5 elo.
A second player leaves team 1 ->20/3->6.6 elo +-
Ok maybe it should be restricted on only the 10 devided by staying players if its implemented. So that you can't gain +elo if u loose. Maybe something like:
(start players - leaving players team 1 + leaving players team 2)*elo gain/ (startplayers-leaving players team 1)
Back to the example with dota and 10 elo gain/loose with 2 leavers on 1 side and 1 on the other:
(5-2+1)*10/(5-2)= 4*10/3= 13,3 elo win
And if we substract starting elo 13,3-10-10=6,7 elo loss
If this is already implemented in ElO calculation please excuse me the time you needed to read that

And of cause numbers and formula could be changed just a quick idea.
greetz FTHGN