stringstrings replacement for XeTeX?

Question

To construct a lower-case filename out of the mixed-case argument given to a command, I used stringstrings in LaTeX.

Now I switched to XeTeX and it seems the fontspec-package has some interference with stringstrings:

\documentclass{article}
\usepackage{fontspec,stringstrings}
\begin{document}
  \caselower[q]{AbC}\thestring
\end{document}

Produces an error message only when fontspec is used:

! Missing control sequence inserted.
<inserted text> 
                \inaccessible 
l.4 \thestring

Just using \lowercase for display won't work, because I need to use it like

\edef{\filename}{stuff/\thestring.tex}
\input{\filename}

(there seems to be a similar clash between stringstrings and polyglossia)

So: what would be other string manipulation packages for LaTeX and XeTeX (coolstr is mentioned, but only supports substring operations I don't need)? Or is there a fix by redefining something?

Answer 1

Fixed in TeX Live 2010, thx Philipp.