Jeremy,
the PC Super-Angulon 2,8/28 has indeed a 67mm filter thread, but it is not very easy to use. First, using 67 mm filters would always result in some vignetting.
The thread is in fact exclusively designed to take Schneiders own filter holder and lens hood. However, you can use a step-up ring in combination with a larger filter. Personally, I use a B+W 82 mm slimline polarizer together with a B+W step-up ring. Vignetting only occurs when shifting the lens more than ca. 6mm. If you want to avoid vignetting at all (including shift), then you would have to go with the Schneider setup.
You should equally be beware of the fact that the gap between the filter thread and the lens barrel, which is embraced by the filter thread, is very small. I tried several step-up adapters (hama etc.) which would not fit, being too large. I was told then by Schneider Kreuznach that only the B+W (a company they own) step-up ring would fit. As they said, this would also apply to filters in general.
I never tried Contax filters or hoods on the lens, as I my preferred filter brand has always been B+W.
So it looks as if it boils down to only two options:
1. the original Schneider setup (which also uses B+W filter glass) with no vignetting at all(as they claim)
2. A setup of a large slimline filter with a B+W step-up ring, causing some vignetting in extreme shift positions.
Nonetheless, there is a third option (although it may sound a little dubious): Apart from the polarizer, I use P-series Cokin filter glasses and just hold them in front of the lens. As long as you shoot from a tripod, there's no problem with doing so. And you don't have to carry around the bulky filter-holders together with the various threads. Not only do you save space, you also save time for screwing/unscrewing, changing adaptors, glasses etc.
This is what worked out best for me, maybe others have more experiences with other filter brands.
Hope this helps,
Marc