Some of hte ideas from http://www.w3.org/1999/04/Editing/ might come in handy there. What realistic usage scenarios are there for multiple clients concurrently uploading (parts of) the same file?

So far we have this:

> Servers MUST handle overlapping PUT requests in an idempotent fashion given that the overlapping data is identical. Otherwise the behavior is undefined.

Does that adress your concern?