Attachment Security Modification?

[Sorky]

I need to provide a small file modification service and I hope to use the attachments in a sub-forum for this. The idea is for ANY user to create a thread and post a file that only a specified user(s) will be able to access. They will then post a reply with a modified file and I need the thread creator to be able to download the attachment, but ONLY for that thread, so I do not want to add them to the group that can download from that forum.

A bonus would be if the thread creator or moderator could somehow add other users as co-owners so they could also get the download (but only once they had special permission). All of this of course should happen without the need to use the back-end ;-)

Is this sort of thing possible? Does it need a modification? Any ideas appreciated.