I just updated wordpress of this add-on to the newest version on dlink320l on firmware 1.10.
To do this, install the sshd add-on, start it and login with putty with root access (default username is root with blank password, so make sure you change it immediately). Stop, the wordpress service and then download the newest version of wordpress. Replace the folders wp-admin and wp-content from your downloaded copy to wordpress/www/. Afterwards, overwrite the wp-includes folder and the other files which are left. You can then start the wordpress service again and you will only have to wait for it to update the database. Mine was a blank wordpress, so I'm not sure how this solution will impact one with content in it.