<xsl:function name="bib:footcite" as="xs:string">
    <xsl:param name="footciteable" as="element()"/>
    <xsl:number level="any" select="$footciteable"
       count="db:footnote|db:citation[not(ancestor::db:footnote)]"/>
</xsl:function>

...which ought to work, right? (Bruce will probably tell us if not.)