Hehe, it's funny because this used to be a reason that I would not play the slav, or why I considered giving up the french. I don't know, I think in general it's probably more of an irrational fear. Sure there's always the possibility that someone will play the exchange, but rarely does it actually happen. There's so much incentive not to play it from white's point of view, since in general it doesn't challenge black, so from black's point of view there's not much reason to expect it.
I still tend to think chess is chess. You can start out with a symmetrical looking position but still make better decisions than your opponent from that position. Even in an endgame, there will be decisions about where to place your pawns, what targets/outposts to create, etc, all areas where you might outplay your opponent.
Let's put it this way: has the slav exchange actually been a problem for you? It hasn't been a problem for me; I've played the slav for about a year now and no one has ever even played it. If someone did I would just play principled chess and wait for my moment to outplay him. I don't think I need to change up my whole way of playing just because someone might offer me a position where black has no problems equalizing. That can often backfire anyway -- if you are unfamiliar in a position, that can allow a lower rated player to give you problems that he would normally be unable to. I've been playing the french for over 5 years and my opponents have not done particularly well against me when playing the exchange.
To summarize, it's probably not worth putting too much fear into this. It rarely happens, and symmetrical doesn't have to equal draw. Even Carlsen wins symmetrical looking positions from, say, the QGA. Perhaps if you know your opponent plays the slav exchange you could prepare in advance, and I'm not saying having a "backup" system is a terrible idea, but I would consider it quite optional really. Besides, playing for two results right from the start, as black, is a pretty nice deal, even against a lower rated player.
...Or do you even play the Slav against inferior opponents?
I had the black pieces against a 1700 this weekend who played 1. d4 and 2. c4, and I spontaneously decided to play 2...e6 instead of the slav, since I was certain he'd go for the exchange-slav. But this would have given him the oppertunity to play the Catalan or something I'm not familiar with. Luckily I was able to transpose the game back to a semi-slav, but it got me thinking...
How serious of a drawing weapon is the Exchange Slav? Obviously it's not something I'd worry about against someone with a rating much, much below mine, but I feel like players in the 1800-2000 range could make it very hard for you to win on the black side of the Exchange Slav...
Here's an anti-exchange slav line I've looked at, but I decided over the board that I didn't feel comfortable enough in it.