Remapping German sharp s (Eszett)

I assume mine is a general problem with remapping, but what I want to dois to use, with Linux Libertine (and XeLaTeX), the round (ligature ss)form of German sharp s (E04C) instead of the standard form (00DF).

I thought this could be done by adding

U+00DF <> U+E04C

to my tex-text.map.

This does not seem to work however. Compiling

\documentclass{minimal}\usepackage{xltxtra}\setmainfont[Mapping=tex-text]{Linux Libertine O}\begin{document}Lessingstraße\end{document}

with XeLaTeX still gives me 00DF in ``Lessingstra_ß_e''.

Can someone please tell me what I am missing here?

Thanks in advance,

Rainer