Some thoughts:
- every further external dependency makes DC more complicated.
- as adbenitez says: Sending large files is not a good idea in slow networks.
- How stable is a direct p2p connection? I think sending reasonable file junks by mail could be the more stable solution.
- splitting files for sending them by email is commonly used with standard MUA and there is not really a problem when using this approach if servers have the space and bandwith.
I think the way to split files to several mails is the easiest approach because all technology is existing in DC.
A first approach could work without sync mechanism. This would at least serve for people with big enough space at server.
… and is usable by standard MUA too in a manual way!