I was just wondering about the intention.
About the "magic": xowiki offers a higher flexibility for using template files compared to other packages. Furthermore, while most packages on OpenACS use purely function-based templates (e.g. "edit"), xowiki uses an object oriented resolution based on objects and methods (the template for "edit" for different pages or types of pages can be different).
The methods responsible for rendering are defined in xowiki-www-procs.tcl. The method for viewing pages is the method "view". xowiki does not pass blindly global variables to the template, but names explicitly all variables passed to the templates (see the invocation of the method "return_page"). This is done actually to to reduce the level of magic. For every method you find there all variables that ones can depend on in the template file.
I will add the passing of the property as you suggested to xowiki. Do you have an example, where we can the included "portlet" you are mentioning in action?