<xsl:param name="username"/>
<!-- .... -->
<xsl:template match="btr:attribute">
<xsl:attribute name="{@as}"><xsl:value-of select="${@name}"/></xsl:attribute>
^^^^^^ - how eval this variable?

CHange the param to

<xsl:param name="params">
  <username>....</username>
</xsl:params>

and evaluate it as $params/*[name()=current()@name]