Dave,
I just picked up a 500mm II a few months ago and did extensive research on Canon's drop-in polarizer. What I found is there is no better choice than Canon's polarizer. I suspect that companies like B+W (etc) don't make a drop-in circular polarizer for the super-teles based on the overall demand. They probably would not sell enough of them to warrant the engineering and production costs that come with designing/producing a new filter.
Personally, I've found Canon's drop-in polarizer to be a great tool and I've used it several times in the field. Specifically, I find it to be invaluable when shooting scenes with water. I've used this lens to shoot waterfalls as well as birds that are on or near the water. In both cases, the polarizer allowed me to create shots that I could not have done otherwise. The polarizer will also come in handy when shooting landscapes with clouds in them, but I have not come across that particular situation yet. (Yes, 500mm can offer a very unique perspective for landscapes!)
If you are only shooting birds/wildlife that are not near water, or using it for sports only, then the drop-in polarizer may not be of much use to you.
That's what I've discovered so far -- hope it helps!
Bryan