iRule to send multipart file upload to a different pool

When you use the term "chunk" do you mean true HTTP chunked transfer encoding? Or is this something custom-built the application itself does to break the large file into separate 10MB pieces, then send over separate connections? If multiple connections, you might be able to incorporate some sort of persistence. If there's any part of the payload that indicates this is one of those special large file uploads, you could use that with a universal persistence profile and an iRule to force the connection to persist to the same server as previous connections.