From what I've read, they pretty much instantly react with chloramines, then get boiled off. If you are really worried, you can put in the campden 24 hours in advance which will allow more to dissipate before the boil.
The reason sulfites are more of a problem in wine, is that they are added during racking to stop oxidation. Added that wine is never boiled, the sulfites never really dissipate.
Also, from what I've read, ascorbic acid can be used to prevent oxidation as it is a good anti oxidant, but I've never heard if it being used to dechlorinate water.