Hi Darren,
It may be very different than what you're doing, but the hybrid
approach of gnuplot's 'pslatex' terminal device might be worth
considering and looking into. This writes postscript for the graphics
part and leaves simple lines (for the axes) and the text as plain
latex that overlays the postscript. I think for mpl, you might want
to have all the non-text go into the postscript and all the text go
into an accompanying latex file, but the idea is the same.
One advantage there is portability, as the output is latex+embedded
postscript (using \special). It also allows pretty fine-grained
control on the output, including changing fonts or doing latex things
that mpl can't do (and this can all be done after the fact, so that
you can create the figure, and then change the fonts). It does
require latex to create the figure, but this step could be automated,
at least to stage of the dvi-with-eps-figure stage. Getting to ps,
pdf, or png would be less easy to automate but may be doable in a
portable way.
That sounds easier than parsing a dvi file to me. And postscript
backend already exists.
--Matt