Thanks, Christine. The Sig 10-20 does have filter threads (77mm!), so I assume they left a bit of wiggle room. If I see vignetting, I'll be rather annoyed, but Hoya specifically says these are low-profile, so I'm not expecting any serious trouble.
I expect later on I'll want to try a graduated ND and/or CP, which... at 77mm... is no small investment
Edit: Sigma states that any filter thicker than 5mm will cause vignetting on this lens; reportedly the DMC Pro1 is 5mm thick. Several reports online of people who use the 10-20 with 'normal' filters, and see no vignetting.