If I set a custom document field I can use that, but then I lose drag n drop functionality...at least I think. Any ideas?

you must create a workflow action script if you want to include this feature in a workflow, if not just create a normal script for this purpose.

I believe if you create a text area field you can drag and drop attachments in there. You'll see the link in the field. The only caveat is that the file is not public so you will need to go into the filing cabinet and set it to public.