I have thought about this myself. At first I considered buying an underwater enclosure as this would surly work but these are very expensive. I have been wanting to take some panos from shallow water and in the surf...etc so I can think of many ways to make use of this. But they cost about $500 to a $1000.
I have also wanted to take a pano in rainy weather.
I have thought of building a square box enclosure out of lexan. I would simply extend the tripod mounting bolt through the base of the box. The base would be made of wood. This way you can then mount the camera inside the box. The box and camera would be mounted to the rotator which is mounted to the tripod. You want the lens butted up next to the lexan as close as possible. The back of the box has a door or open space through which you would mount the camera. If left open you could cover this with plastic that is velcrowed to the back. Operate the camera using a remote cord or wireless. The whole box with camera inside rotates because the rotator is mounted on the tripod under the box.
This is not "water proof" but water resistant as they say. But I think if properly built you could use it without an umbrella in the pooring rain. If you get fancy you could build the back door to be water tight using rubber seals and a screw mount cover for the back.
Some of the box corners would get in the image but you could photoshop this out later.
The lexan box is how the famous "fish in water" pano was made. There is a thread here but is old about this pano. Some guys from japan put a coolpix 950+FC-E8 lens into a lexan box and partially submerged it in a running stream. The top was open and it was done hand held but I figure if they did that then one could do the same thing for rain.
I would build one myself now if I wasn't already working on a custom flash bracket for use in pano work. Its off the subject but I think I could pull off using a flash with my 10.5mm but the flash must be mounted verticle over the lens.....anyway.
Just some thoughts.