For dynamic publishing, it looks like text filters are not directly supported. Instead, if a text filter is specified then a global filter with that tag is used.

Global filters are modeled as Smarty ‘modifiers’, which turn out to not get access to a global context object (bad design on the part of Smarty, IMHO). However, the API notes for MT have the following statement:

If you require access to the $ctx variable, you can access it through the global “$mt” variable, which has a method named context that returns the template context (Smarty object).

That should suffice for the caching needed by WikiVar and Webiki.