This is actually "almost" supported in 2.2.0
Basically if you do something like:
... some stuff here ...
<@pg.applyDecorator page="/some-other-page.ftl" name="some-other-decorator" />
in a freemarker decorator and the main result page you're decorating is a jsp page, it works.
However, if the main result page you're decorating is a freemarker page, it doesn't.
When you try to run it on a freemarker result, you don't get any errors, it just skips the content (possibly ignoring applyDecorator altogether)
I've also been able to confirm that when trying this with a freemarker result, the JspTaglibs and the page.applyDecorator tags are available to freemarker