> So I assume that current() results in an xs:string,
yes well it would if it ever got that far:-)

      <xsl:apply-templates select="/

  / means the root node of the current document and that means
  root(.) instance of document-node()
in your case . is a string so die as root() needs a node.

as always for preserving context in a for-each

<xsl:variable name="doc" select="/doc"/>
    <xsl:for-each select="tokenize( 'a c', '[ ]')">
      <xsl:apply-templates select="$doc/*[current() eq string(@id)]" />

David