Don't think I'll be able to feed a cloud-stored file to an SMTP server, directly, without downloading....
But -- I just played with it a bit, and compared to DropBox, here is what I see:
1 - When adding an attachment from AquaMail, DropBox is listed, Google Drive is not
2 - When choosing a file from DropBox, it's automatically downloaded (to some sort of temporary directory, I presume), and then control is returned to my app, which is able to handle it like any other attachment
3 - Opening a file in Google Drive, and using Share seems to be the only way to include it in a message. A filename with Russian characters was messed up (in native Gmail app, too).
Seems like Google has work to do, to bring sharing in Drive on par with DropBox.