This is how to do it:

1. Open the seat selection page of your flight.

2. Open the source code of this webpage (for IE, click "page->view source")

3. Change to any other seat, click "continue".

Assuming your new seat is 20-A.

4. From the source code of the previous page, copy the URL for seat 20-A. open it in the web browser. It looks something like

"https://webx25.nwa.com/cki-bin/cgi_pnrupdate.pro?selectSeat&seat=ae2124350c18136e 91545c...............#seatmap"

5. Now, the disability seats are available for selection. (Unfortunately, not the premium and exit row seats)