For html I tried this with an extension (see attachment). This works for modification of the figurelabels and the references to figures. But: the figures are not numbered correctly, because every document stands for its own and consistent numbering for the all documents is not possble..
So I don't think this can be changed with an extension, but with something like the method "assign_section_numbers" in class BuildEnvironment.
If there is a dictionary for figures, tables and sections (there is one for sections) in the environment, one could use this to change figure and table captions and references.
by the way: thanks for sphinx. I'm beginning to understand it and I learned a lot.
I just want to add that in the current version of 1.1.2 the pagerefs for tables are set wrong. A phantomsection with the label is created before the tabular environment so that all references point to the page where the reference is set not to the page where the table is printed in LaTeX.