If I need to shoot in the cold for long periods of time, I will acclimate my lenses to the outside temperature. For example, I will take my lens case (with the lid OPEN) and lock it in my car or place it in my garage an hour or so prior to shooting. A warm humid interior is the culprit for exterior fogging. Then when ready to march out into the cold to shoot, toss in some silica gel packs like this into the lens case then close it:
http://www.filmtools.com/silgelselin.html
Moisture+Cold=Fog
If you cannot acclimate the lenses, purchase some disposable hand warmers and place them right on top of the barrels (taking and Kowa) if you start noticing fog. Another great use for the hand warmers is to keep your batteries warm(-ish) to get better performance. Wrap the hand grip with one and also toss some into your bag next to the fully charged spares.
As for filters I use both screw-on Series 9 and square 4x4. I have yet to find 4x4 diopters so I screw on the diopter(s) on the front of my Isco and then slide the front of the lens into the donut of a small 4x4 matte box that is set on 15mm rails. Another advantage of 4x4 matte box is that I often use ND graduated filters and a polarizer which just don't work on the front of the Isco (the Isco's focus barrel is the front-most barrel and turns when focusing).
I agree with Bioskop that your 90mm Leica would be perfect. My favorite taking lens is a Nikon 85mm 1.4 AIS. I'm also playing around with a 50mm Nikon Series E pancake that has a bit of "falloff" at 1.8 but not a full vignette that would make me discount it entirely.
Good luck, stay warm!