The Heinlein Archives have gone online now. From the website:
“The Heinlein Archives presents digital copies of the entire collections of Robert A. and Virginia Heinlein in downloadable form for research and academic purposes. Heinleinâ€™s OPUS manuscripts is the first complete collection presented. These manuscript files include Heinleinâ€™s files of all his published works with his notes, research, early drafts and edits of manuscripts, to the final published form. These files provide both a look at Heinleinâ€™s creative process and add insights into his intent and the meaning in his stories”.
Unfortunately the website is not free (as I think it really ought to be) and t & c’s are on the page linked to above.