In the Android App, if a question is closed, I won't be able to leave comments on either the question or any of its answers. There are simply no "Add Comment" buttons available anywhere if the question is closed.
See the screenshot below. The comment adding interface is simply removed on closed questions and their answers — no Reply button in the toolbar, and no Add Comment button under the question.
Occurs on Android app v1.0.50 and v1.0.51, on a HTC One running Android 4.4.3.