Configure per-document LaTeX preamble for rendering formulas

Dear all,

I am very surprised and pleased that Inkscape now supports LaTeX rendering.
 Up to now, I am still using xfig for producing my figures. The reasons
for using xfig are:

 - automated generation of EPS and PDF files using Make, fig2dev, and
metapost

 - possibility to use an own latex preamble this way

The current LaTeX rendering -- although already very nice -- does not allow
me to add a per-document latex preamble which, for example, includes macro
definitions that I later use in the formulas (e.g.
\newcommand{\variance}{\mathcal{V}} and in the formula $\variance =
\ldots$).

Another problem is that the information concerning the LaTeX formula is not
saved in the document, i.e., it is lost after the EPS files is generated.
Thus, even if I were able to include a LaTeX formula (without hacking the
extension), I could not recreate the PDF or EPS generated by Inkscape if
macros in the preamble changed since LaTeX formulas are "static".

Unfortunately, this missing (advanced) feature makes me stick with xfig.
However, I would really like to Inkscape.

Maybe, this feature is also interesting for other users or maybe there is
also a solution for my problem?

Thanks anyway for this gorgeous tool!

Best regards,
Michael.