You don't need a circular polarizer. You only need a circular polarizer if you have another polarizing surface on the image side of the polarizer, like the semi-silvered parts of the mirror in AF SLR's.
Leica makes a polarizer which is a swing out one. You compose after swinging the polarizer 180 degrees, so it's in front of the viewfinder window, turn the polarizer until you have the effect you want, and then swing it back over the lens through 180 degrees. The effect is symmetrical through 180 degrees, so now your lens sees the same effect you saw through the polarizer.
Cheaper way is to get a filter adapter for something like a 72mm filter on a 46mm lens, and then use a dremel tool to make a hole so that you can see through the adapter (figure out where to make the hole after mounting the adapter on the lens). Then you buy whatever 72mm polarizer, put it in the adapter and see the polarization effect through the hole you made. It looks slightly clunky because of the size of the filter, but it works. 72mm tends to be OK because it doesn't block the rangefinder window.
Henning