> Could something like this be compiled into the Templates 
> object? (is it?)
> 
> <xsl:param
>    name="something"
>    select="document('something.xml')"/>
> 

No, at present Saxon never preevaluates the document() function. In fact,
it's about the only function it doesn't preevaluate when the arguments are
known at compile time.

Michael Kay