Next to the object size/position number boxes at the top edge of the Inkscape window, is a selection list which controls which units they are displayed in. We can add another selection list which controls whether the values displayed are relative to the screen, drawing, or export scales.
Maybe, yeah. The window chrome is already so crowded with cruft though, and this feature will be relevant to a pretty distinct audience so I'm uncertain that adding it as another edge widget is ideal. Not that I have a better idea to suggest though...
This will be extremely confusing. The unit selector should always work in document (drawing) units.
Instead of adding an extra widget, we should add the variables "displayscale" and "exportscale" to the expression evaluator in the spinbox, so that writing e.g. "3in / exportscale" would yield a length which corresponds to 3in when exported.
There are other useful constants which could be added, e.g. pageheight, pagewidth, and so on.
Regards, Krzysztof