If there was a "Reply" button on closed threads, then the thread would not be closed.
The only way a member could possibly quote a post in a closed thread would be to manually type the code for the quote. Including the member's username, the post number (which would have to come from the URL from the original post) and the member's number (which would have to come from the URL on the member's profile). If anybody wants to go through the trouble of trying that, here is the code. Insert the member's name, the post number, member's number, and the text. Then remove the spaces from the quote tags, and post it. If you don't make any mistakes, the correct quote should appear.
[ QUOTE="Xxxxxx, post: 00000000, member: 00000" ] Insert text [ /QUOTE ]