Don't know about current versions of the hardware and firmware, but this is what I've done with my Hardware A3, firmware 1.21 Dir-655. I learned about this "trick" indirectly, from a post here from July of 2009! Search for "Bridge mode does still exist on DIR655". Even though you're not looking for a way to enable bridge mode, using the same trick in Firefox will allow you to get to normally invisible settings in the Network Filter screen, which IS what you want.
Anyway, running Firefox in "View:Pagestyle:No style" will get you the extra menus, and going to the Network Filter screen will reveal that you can now enable MAC filtering for wireless clients and wired clients separately. Just deselect "wired client" and you should be able to do what you want. Don't forget to save the change (and be careful as there is a lot of extra stuff on every settings page when you set Firefox to "View:Pagestyle:No style").
Hope this works with your version of the firmware.
Billy