Quote: But if it's an EF-S mount lens ... oh, disaster looms.
Yes, it won't fit.
Sigma (& maybe other indie) lenses designed for APS-C sensors use a standard EF bayonet and do not contain components which might hit the mirror when used in a full frame camera. They're perfectly safe to mount, but will not cover the full frame - the corners will be completely black, not just darkened by a bit of vignetting.