To put this one to bed, I just went out into the garden to get some test shots. I'm pretty sure there are two types of vignetting which take place here.
Please ignore the sensor dust and flare, as the hood was (of course) fully retracted for all shots.
First, there is what I will call
optical vignetting, which is worst wide open, and occurs from MFD to infinity. It looks to be significantly more than half a stop to me (at least at infinity focus, where it almost extends to APS-C frame), but should be correctible in post processing.
MFD f/2.8 Infinity f/2.8
Stopped down, the optical vignetting is gone at MFD, but at infinity there is what I shall call
physical vignetting, where the leading edge of the lens casing (not the hood, which is in all cases fully retracted) impinges into the field of view. Only a crop will fix this.
MFD f/22 Infinity f/22
And a couple of intermediate focal length shots to demonstrate how the two types of vignetting interact.
Landscape f/2.8 Landscape f/22 Bottom line: the Pentax-DA 1:2.8 35mm macro limited is NOT a full frame lens, except stopped down at close focal distances.