Ascorbic acid + Chloramine = dehydroascorbic acid + Ammonium Chloride
C6H8O6 + NH2Cl = C6H6O6 + NH4Cl
What happens to the dehydroascorbic acid? I have no idea.
The ammonium chloride will increase your free amino nitrogen slightly in your wort.
I imagine you will require miniscule amounts. Probably the cheapest and purest forms would be some tablets from your local pharmacy/vitamin supply.
Do you know ppm of chloramine in your water? 2-3? Should be relatively easy using a little molar chemistry to figure out how much ascorbic acid to add. What would the effect be if you added too much? I have no idea. A little too much is probably not a problem would be my guess.
I would be hesitant to add it to other than just the mash water (ie. I wouldn't add it to the hot/cold liquor tank if this water might be used elsewhere).
Ascorbic acid (used as an antioxidant by many brewers who pasteurize) will actually function as an oxygen carrier (vs. oxygen scavenger) in the presence of free metal ions (like calcium, iron, zinc, magnesium, etc). This has obvious implications in terms of shelf/flavour stability etc. EDTA is commonly commonly present in commercial preparations of ascorbic acid as an antioxidant additive. EDTA chelates free metal ions. This allows the ascorbic acid to function as an antioxidant.
My point? I would be pretty comfortable using this in my mash water if I had a chloramine issue. I'm pretty sure (as long as I didn't go overboard) the process/yeast wouldn't freak out much. Using this technique elsewhere in the process would make me think a little harder about it and its possible implications.
Does your carbon filter not deal with your chloramine?