Yes, I have heard of this method and have found a number of jars inside upright pianos (usually empty because the owner forgot or someone else put it there). I think they are certainly helpful, but water only evaporates so fast, so effectiveness is minimal.

William Braid White mentions this in his book (see photo).

I agree that adding too much humidity too quickly to old wood can be problematic. I always suggest 40%-50% for hygrometer readings near a piano.