Hi,
I have Lee and P-series Singh-Ray systems. After I got my 16-35f2.8L II, only the Lee would work with a polarizer, and not lower than 19-20mm at that.
The P-series vignette at anything below about 24-28mm with the sprocket type polarizer.
The Lee system can use polarizers in a few different ways. They can take a 105mm polarizer in a threaded ring mount that sits anterior to the filter slots--this vignettes before you even get down to 24mm. You can also thread a polarizer directly on to the mounting ring that the filter holder itself then mounts to--if you get a thin mount polarizer this will work down to 19-20mm.
Finally, if you buy a square 100mm glass polarizer you can have an optical technician grind it down to 100mm diameter circle that can now be slipped into one of the slots itself. This can be oriented with the required polarization as it is now round. This last way is not as convenient but has no vignetting right down to 16mm.