This draft (intended for experimental status) is ready. I agree with the authors that there is really nothing new at play here, with the exception of defining a "Reaction" Content-Disposition, with a Content-Type of "text/plain". 

The authors have included a Usability Considerations section and point out that certain behaviors need to be worked out in practice (i.e. should reactions be sent to all recipients of a message or only back to the author?).