I created a patch for this about six months ago and discussed it on the TT mailing list. It allows you to write:

[% GETFILTER '::html_escape' %]
  ... all gets are now filtered through the function ::html_escape ...
  Hello, [% name %]
  [% GETFILTER '' %]
  ... turn off filtering ...
  [% END %]
  ... previous filtering resumes ...
[% END %]
[download]

It maintains a stack of filters, and there's no reason why you couldn't pre-initialize it with a default filter.

At the time it seemed to generate some interest, although not enough for me to do much more with it. Perhaps by now some form of it has made it's way into the latest release - I haven't checked.