I don't think we're planning on exactly what you're after, @deviant, but we do have plans to make a change to the flow of adding related items that may work equally well. Right now, 1Password for Windows only has a "link existing" option, but we're planning to have a "link new" option as well that creates the Document item and links it in one step. The item will still be a separate entity, but you'll be able to upload it and link it all at once making this process at least better mirror adding an attachment.