I used too much maltodextrin in a chocolate stout, and it feels almost like drinking syrup. Is there any good way to mitigate the thickness it imparts without watering down the flavor of the beer (too much)?
If it's physically thick, I think the only method would be to add water. At the far extreme, you could make another batch that's a bit watery and mix them.. How much did you add / how much did you mean to add?
If the beer is in a keg or fermenter, you could try adding some purified amylase. You might be able to find it at an online homebrew shop. Amylases are a class of enzymes responsible for breaking down starches in the mash. Residual maltodextrin should be able to be digested the same way. Primarily by Beta-amylase I think.
On option may be the use of Beano if you can't get purified amylase extract.
The only trouble with adding these things is that they tend to be tough to control as far as the end point goes. Often you end up drying out the beer too much.
Another option would be to pitch Brett or some Pediococcus. These yeasts can utilize dextrins as fuel sources better that normal Saccharomyces. So if you don't mind having something slightly soured and funky that might work well too. Depending on the amount of maltodextrin left that will dictate the amount of Brett or Pedio character you end up with. Done right it could be pretty nice.
If the beer is bottled already, I think you are out of luck. Start blending it with other beer if its that bad.