Maybe I am old fashioned but I just don't understand the problem. For as long as I can remember scale drawings said in the legend something like "1 inch per foot" or "1 inch per mile", or whatever. In Europe it was probably "1 cm per meter", which is even easier. Then the person making the drawing, and the person reading it, would just replace one unit for the other in their head - 10 was inches on the drawing, but feet in the real world. Which was pretty easy to do so long as it was 1 of these for 1 one of those, and only slightly harder if it was 1:5 or some other ratio.
The document being produced by Inkscape is a drawing, and it will never be at actual scale for microscopic or macroscopic objects. If it was it wouldn't be possible to print the drawing without scaling it up or down. This has always been true of drawings of actual objects, except in a few extremely rare instances where full scale drawings are produced (like masks for chips or spray painting templates).
Would not some sort of "sticky" comment that reflected the drawing to represented object length ratio suffice to address this "problem"?
David Mathog mathog@...1176... Manager, Sequence Analysis Facility, Biology Division, Caltech