I am still not with it. I have set my Qumi to no overscan, the HD-FOX driving it to Auto, and I have Breakfast content showing the usual variability in picture width (with the title bars extending to the full 1920 pixels on HiDef). From what I gather from selectortone's posts, I should be seeing the horizontal width being stretched in Auto to fill the display frame (with some non-linearity in the stretch), but I do not. The width variability remains.
In fact, I have also noticed vertical variability according to source material (less frequent than horizontal, and despite the 'FOX setting to Auto and there being no title bars to define the full height).
In my opinion, this is (at best) "unproven", and presuming the Humax is not attempting to fill the display frame dynamically I see no opportunity for it to introduce linearity issues. When I get a few minutes I will knock up a couple of test cards with non-standard pixel sizes.