Not possible to have 1920x1180 multiimage?

I'm trying out something with parallax scrolling that uses 1920x1180 wallpapers on a 1920x1080 screen. For whatever reason, though, those wallpapers are being shrunk in proportion to the 'overdraw' and rendered either as a bordered image (if aspectratio is set to 'keep' or 'center') or an upscaled one (if aspectratio is set to 'scale'). This has the weird of effect of drawing the images as I want them, only at noticeably lower quality due to the upscaling.

Any ideas?

EDIT: I've canned the idea anyway, but will leave this in case the answer happens to be useful to someone else.