try putting xml:base at the level of the stylesheet where you need it, e.g.
on a template rule, and have a global variable

<xsl:variable name="stylesheet-uri" select="static-base-uri()"/>

The global variable will then use the "real" base URI of the stylesheet
module.

<xsl:template match="/" xml:base="foo">
	<xsl:value-of select="document-uri(document(''))"/>
</xsl:template>

With xml:base on the root matching matching template, why does calling
document('') return that base (the same goes for xsl:stylesheet).  I
would've thought the root *node* returned from document('') was
outside the scope of the xml:base change, and so would return the
original base...