Yes the Pany 7-14 can take filters!

...with a bit of help from a peanut butter jar lid and an old unknown brand filter holder...

I know this has been done before, but here's my adaptation and allows use at 7mm without vignetting - I had an old unknown brand filter holder with a 77mm thread. I was going to buy another lens cap, but fished around and found a plastic peanut butter lid that when I trimmed the thread away made a nice snug fit over the fixed lens hood of the 7-14 (that's the flash of blue you can see) and which allowed a similarly snug fit inside the 77mm filter thread with a round or two of pvc tape. A few dabs of super glue for safety and voila. It fits to the lens by friction and I can push it back far enough that there's no vignetting even at 7mm so long as I have the filter holder nice and square.

I took it for a spin using a glass Lee CPL and a 3stop soft Lee GND. Not the ideal scenes for the GND, but it was more for practice and at least show's the rig works.