In an old experiment program we used to use, there was a way to define the "anchor" of a visual rectangle. It used compass-point symbols (C N NE E SE S SW W NW), but it could also be defined graphically. What they mean: "C" means the center of the image rectangle is positioned relative to the center of the display (more or less what SuperLab has now). "N" means that the middle of the top edge of the image is positioned relative to the middle of the top of the display. "NE" means the upper right corner of the image is positioned relative to the upper right corner of the display. And so on. How this is different and more useful than just moving the center of images around relative to the center of the display (or to any other single point) is (for example) that it allows you to put something exactly at (or a certain distance in from) the lower-left (SW) corner of the display regardless of the size of the display. That's something that otherwise, you'd have to adjust for each computer and each display resolution you use. If you allow the use of fractional coordinates (that is, for a scale resolution of 1/1000, scale a horizontal coordinate h by width*h/1000), then you can get even more display independence.
Of course, the SL scenario file would have to be modified to represent this feature, which I'm sure would be a pain. However, if there's even a one-byte hole in the specification of stimulus locations, you could add this with only five bits: 1 to enable scaled mode, 1 to enable the compass point logic, and 3 for the 8 compass points. All zeros would mean unscaled center mode, as it is now.
On the other hand, maybe there's already a way to do these things, which would be wonderful. (I'll ask the question in the support forum.)