The only research I know of on the subject is Muriel Harris' article on
one-draft and multi-draft writers, in either CE or CCC. Also, there's the
Jensen stuff on personality variables and writing process. Ultimately, some of
us draft in our heads and some on paper. Even my freewriting has punctuation,
caps, correct spelling, and I do a lot of editing, just backspace/deleting, in
email--habit I guess. If I freewrote on the computer, I think I'd do the same.
I basically don't do much hand-writing when I'm doing writing anymore. I
suspect the switch to the computer significantly changes the nature of what we
call freewriting now--it inevitably looks more like "real" text.