I dunno, but you won’t get it from the twain specification. Could be that some vendors build a driver with a custom file format (adding raw, pdf, etc.. ), there is a lot of room in the specification for vendors to do custom stuff, or it could be that you got peudo-raw (a bmp converted to raw – which for me is more medium than raw, but whatever). If you know someone that got it done, try contacting them.
About you wanting to know what vendors support {insert your hearts desire here}, yeah that’d be superExtraFantastic if a list like existed. It doesn’t, sorry. The closest thing to a list like that is a site maintained by Dosadi, where they host their twister archive:
http://www.dosadi.com/twister-reports.php
.