I just got the SD card I ordered, & installed 9.04 on it from the live USB I had been using (unfortunately, the persistence doesn't carry over, so my new, clean install needed the same old tweaking). Anyway, I got the Fn+2 shortcut to work without installing Ubuntu Tweak.
In Keyboard Shortkuts, I cleared the Next Track shortcut, then set /usr/bin/aircraft-manager to XF86AudioNext as above, and it didn't work, as above. Then I set it to Ctrl+O, and it worked! So I set it back to XF86AudioNext, and it worked!
Now, Fn+2 brings up Aircraft Manager without extra software installed.