Oh, I know where to specify a custom origin point; what i'm saying is, when I do so, those numbers seem to be being ignored. In fact, even if I switch the preset values (eg. change from 'bottom center' to 'top left') I am seeing no change.
I'm beginning to think now that it's related to the issue of the collision box coordinates *sometimes* not updating in an animation's collision tab when you manually punch them in.
Adding blank space is not an option, as my characters are quite hi-res, and the resulting sprite sheets would be hideously large.