I don't think there is one. There is or used to be a screw in filter holder that you might be able to put two filters on. I have never tried to do that so it may not be possible. I am not the right guy to ask though as I don't use filters of any kind very often. The only exception being a protecto filter but not even that on my 600mil.
Yes, I agree, but I will definitely rotate the polarizer. There is a Canon 52mm drop in with polarizer and a very small wheel to orient it, that canon sells. If I mount a 4x4 inside the hood, I will rotate the entire hood which would be a little clunky, but at least I'd always know the absolute position of the polarizer and could adjust quicky (Not as possible with the drop-in polarizer).
I have seen 3 and 4 stop polarizer filters, but just in 77mm size. I might have to cut out a 52mm circle from one and replace the glass in the Canon drop in polarizer filter. Would be nice if canon sold one. Or maybe ther is enough room to glue in a 52mm ND with the 52mm polarizer?
I could have sworn that Canon does make a 52mm drop in filter that you can put a 52mm filter in. At least it was for version 1. It comes with a clear filter but you can unsrew it out and replace with other 52mm filter. Some filters are too thick and won't fit. They also have one that holds user-cut gelatin filter. Which 600mm lens do you have (ver. 1 or 2)?
Canon does have Gelatin and screw in drop ins for 52mm, but I would lie to achieve polarizer and a total of 3 to 4 stopd. The filter holders are not designed to hold more than one filter at a time, I I have not found a 3 stop or 4 stop polarizer in 52mm.
I don't have any experience with Gelatins. My intuition tells me they are not as good as glass? if only because they are harder to handle and keep clean out in the wilds?
"You can't have a drop in since you have to rotate it for effect."
The polarizer has a knob on the outside that allows you to rotate it. It is not a cheap filter believe me. They did have a 52mm screw in filter holder. I have one but I don't have ver2 fo the lens so I can't try it.