It isn't clear from the design doc, but it sounds like we're proposing to develop this from scratch as a set of Python scripts? Are there really no existing software tools that we could re-purpose for this? Something based on WebDAV or its successors? Why not Globus Transfer?
XSEDE shouldn't be developing stuff that has to be maintained long-term when there are other options available. It's disappointing to me that we wouldn't simply do some googling and find an off-the-shelf tool for "collecting files from multiple sources".
I think it would be very easy to use Globus Transfer for this. Bring up a GCS server on the host and set up shared endpoints for the various incoming directories. SPs register & authenticate with XSEDE, we add them to groups (either on their request or by invitation) with permission to their shared endpoint, and they use the Globus CLI to upload their files. The CLI can be automated as easily as scp. A significant benefit is that we are using XSEDE's user identity/auth mechanisms rather than falling back to ssh keys.
For the server-side stuff (moving from incoming to repository directories, generating usage reports, etc.), surely there are existing software packages that do this? What about XDMoD? It seems like a very common use case for which there surely must be off-the-shelf options available?