How about using diethyl carbonate, and possibly a trace of acetic or tartaric acid? The hydrolysis should give you carbon dioxide and ethanol - more reactive than the ethyl chloride and the sideproducts are not only non-toxic, but might even give you a nice sparkling wine!
But if you are allowed to use an organic molecule as a carbon source, why not just use ethanol?
Thats actually an excellent idea.. why not have the flavoring agent and ethanol in the vessels themselves, somehow absorbed into the porous portions of a ceramic vessel that was only glazed on the outside, having been absorbed from the previous strong wine in the container. After all, the wine produced was a refill
and the container held ethanol earlier in the night.
People at the party were impressed that the "best wine" was being served after a less yummy wine, when it was apparently normal to serve the cheap wine after everyone was drunk. Along the same lines, Wikipedia claims Pope Urban 2 thought that the wine from Champagne was the best in the world (carbonate rock filtered), so considering that champagne is made by some sort of process involving filtering with porous rocks, it stands to reason...
1. That ethanol and good flavors absorbed into the ceramic from the "strong wine", but the nasty tastes were left in solution.
2. Later, the addition of water caused the ethanol and small-molecule flavoring agents to seep back out.
3. Because the "strong wine" was used initially, maybe the filtered, but diluted result was still concentrated enough to be considered wine.
4. If not enough ethanol remains anyways... this could quite likely be popular with the modern interpretations that suggest the use of grape juice as a sacrament anyways, and that the people at the party remained jubilant (normally attributed to Miracle) - those continuing to drink heavily may have only consumed enough ethanol to keep from getting that grumpy feeling drunks get when they start sobering up.
5. Something likely killed or absorbed all the pathogens in the water at the time; there are so many possible ways this could have happened. Maybe the water was boiled the day before. Maybe there was enough ethanol. Maybe something else in there had a bactericidal effect. Either way, if nobody got sick the way people sometimes did at the time from drinking water, it probably made a strong impression.
* There may have even been some less than healthy, but delicious lead acetate introduced into solution from the ceramic glaze as well.