Actually, looking at the FileUploadTag, it looks like the only useage mode for this is to have a byte  set directly in the component the tag is bound to.
Since the tag itself supports working with a tmpfile, I'd prefer to be able to refer to the uploaded data as a reference to the file, or at least be able to get a handle to the InputStream (so I can write it to my own tmp file).
First, am I reading the source correctly? And does this seem like a reasonable request?
I'll go ahead and file a feature request in Jira and reference this thread.