I'm not sure on this one.
The HTML/XML of the template determines the PDF's layout, and the freemarker data objects determine the PDF's content (along with any static text you put in the BFO tags for your XML).
Because of that, I don't know if you can use the freemarker data to inject BFO tags - I think they get parsed as content, not layout.
Someone correct me if I'm wrong.
I do remember seeing something about being able to include blocks of HTML/XML/BFO though, but I think it's designed to get those from files, rather than from fields.
I don't want to tell you that you can't do this, I have never tried. Has anyone else gotten this to work?