MythTV is not currently controllable via the airmouse. I am sorry.
It is one of the reasons I am doing a systematic port of all Myth UI bits to Orbiter, so that it can be controlled by virtually every single orbiter in the house (and by any remote that an on-screen Orbiter supports, such as the Go Mouse).
With that said, you may want to go to the 0810 alpha release currently in development. I have been making many changes to the UI which can make navigating with a Go Mouse much more pleasurable (I use one regularly, myself, alongside my Gyration MCR remote, and alternate between the two for UI testing, along with all the other supported devices that I have to do UI testing.)
-Thom