Feature summary (what you would like to be able to do and where):
Currently, Special:MyMainPage redirects to a user's test wiki's main page (or the actual main page if no test wiki is set). There is no way to link to a specific page in a given user's test wiki, however. With subpage syntax, one could do Special:MyWiki/Pagename to go to the page Wp/xx/Pagename on the user's test wiki.

Use cases and benefits
This would be useful in e.g. help and documentation pages that guide users on how to create pages, templates, modules, etc.