[XeTeX] Use of Unicode fonts in LaTeX

[Bruno Voisin]

Is anybody here aware of a standard way (hopefully an already existing 
package) for redefining some LaTeX internal commands such that they 
will make optimal use of all the character set in a Unicode font?

More specifically: if you try to insert some non-alphabetic characters 
in LaTeX, by default LaTeX will often take these characters from 
Computer Modern instead of using the Unicode font specified (in XeTeX) 
by the user as the text font (like Lucida grande). For example, if you 
try to write \textasciitilde for inserting a tilde in a URL, the 
default LaTeX definition is used, namely:

	\DeclareTextAccentDefault{\~}{OT1}                  (from latex.ltx)
	\DeclareTextAccent{\~}{OT1}{126}                    (from ot1enc.def)
	\DeclareTextCommandDefault{\textasciitilde}{\~{}}   (from latex.ltx)

and you end up with the tilde being the corresponding tilde accent from 
Computer Modern, and hence above the surrounding alphabetic characters, 
like:

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If however you take care of redefining \textasciitilde to use the 
corresponding Unicode character slot, by writing:

	\DeclareTextSymbol{\textasciitilde}{U}{"007E}

then the output looks noticeably better:

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The same kind of thing would occur for many characters, like 
\textasciicircum, \textcopyright, etc.

Does it need to be done manually in each case, as above, or are there 
existing packages that already do this?

Bruno Voisin