Make a table in memory:
player, team, multi
(* number of people possible on server)
on start of round:
clear and fill the table
on end of round:
if player is in table and the team (taken from the table, not from current team) won player keeps multiplier
I don't see how you'd be able to abuse it, disconnect and reconnect you still loose multi if your team looses. You can go to another server but it starts with x1. If you don't reconnect before new round starts you loose multiplier. How would it be abuseable? Well, you could alt+x to avoid -1 to k:d and come back again just as you now can go to spectator.
And you loose all xp from ongoing round. Anything else?