As for upside-down mounting, I'd guess that the thrust bearing is for normal mounting only. I'd look to see if you can hang the antenna below the rotor with it right side up.

Often, the mounting method allows the rotating mast to pass through the unit, meaning that you can clamp the top of the mast, and mount the antenna below it.

I'd need to see one up close and personal. Anyone have the installation instructions handy?