I am not sure about this cause I haven't tried it myself but I think you might be able to set at least part of the directory in the database.
1) you set the path to "./" (which is the document_root) - Keep in mind that with this, any file uploaded will be on the document_root directory until you move/delete it
2) you do a callback_before_insert/update that moves the file from your document_root to the folder you actually want.
3) In that same callback, you prepend that directory to the $post['your_file_field'] variable.
That may work. As I've said, I haven't tried it myself, but if it does the trick for you...
The other option (I can think of) is to actually modify the GC library file.
Quick question: Why are you trying to save the upload path to the database?