>     <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]

Michael Kay
http://www.saxonica.com/