How about using two separate loops and connect them with one of these:
http://lytron.com/standard/he_liquid.asp
That would preserve the purity of your waterblock coolant while allowing you to use tap water for the bong loop. These have an extremely low thermal resistance, so you don't have to worry about an extra thermal gradient.
Then again, they are a bit pricy, starting at $159, but I suppose you could make your own liquid-to-liqiud heat exchanger fairly easily.