i am currently trying the latest version 1893f but the problem is probably related to all versions of Stylus Studio. Saxon-HE requires the registration of extension functions using the Saxon configuration. it would also support using a Initializer but only when running from a command line - which isnt the case.
if that could be supported (i.e. using a config file with the functions/classes to be registered, or the option of using the initializer) that would be a buying argument.