It is impossible the way things are currently architected.
I'd have to create individual multiports for each user. It's a feature I have considered adding in the past, but have not had time to fully investigate what it would take or decide on a design.
In other news, I am reconsidering adding per-coin autoexchange to BTC, and doing it a bit differently than the other pools. Stay tuned.
I'm far from a coder, but it seems to me that it would be quite complicated. It would also be nice if, in addition to turning off coins you're not interested in mining, you were able to tune the profit algorithm to put more weight on either the difficulty or the sell price, whichever is more important to you. I'm excited to see what new features you add in relation to an auto-exchange, but it's probably nothing I would use - I use multiport to mine the coins when difficulty is low and just save them, so I have very little interest in an auto-exchange.