The special http namespace is non standard. It requires a dependency on libapreq2 which is tricky to build and the plugin system is otherwise unused.
To support the use case of the http namespace it would be better to support passing parameters (accessed by xsl:param). There is an old patch in the bug tracker supporting that (http://issues.outoforder.cc/view.php?id=74) but it should really support optional quoting as the nginx xslt module does with xsl_param/xsl_string_param.