[XeTeX] Change font metric?

[Karl Berry]

    > I would like to adjust the font metric of this particular character.
    > Is it possible?
    Only by using a font editor--such settings are built into the font. 
    This is not the kind of thing that any page layout program or word 
    processor can do.  

For the record, LuaTeX can in fact do this (although this is not going
to help Thierry -- you can't just run "luatex" instead of "xetex" and
expect everything to work the same).  In general, you can interpose
arbitrary Lua code between reading the font file off the disk and what
gets passed back to TeX.  Hans and Taco make extensive use of this.
It's discussed in some of their articles.

    And, of course, it may not be legal to modify the 
    font; you must check the license agreement.

Indeed.  Although in the case of LuaTeX, the font is not being modified.

Best,
Karl