Kantors generally don't like coins. It may be not easy to find one which will accept it. But try to ask, maybe I'm wrong. But they may also want to lower the exchange rate for coins. You should change it near the Czech border earlier - there might be easier to do that..
Try here: kantor-zoliborz.pl
They do mention CZK coins on their site however you might ring them first since kantors can be unpredictable and expect to get a worse rate than you would for coins.
Easy to find on ul. Popieluszki (click on the link called 'mapa dojazdu' to see exactly where) easy by tram or a very brisk walk from Metro Plac Wilsona.
20CZK is about 3zl max. Why not save them in case you go back to Czech or drop them in the collection box at the airport?
Metro Marymont, by the way is a slightly better bet than Metro Plac Wilsona - not much in it.
That's the only place I've ever seen that advertises that they'll take coins from obscure currencies. I might have a look down the back of the sofa ;-)