Thanks to a pointer provided by Uche Ogbuji, I've just read an interesting paper on "binary XML" written by some researchers at IBM. Previously I've called binary XML the equivalent of broccoli ice cream, and I continue to doubt if it's something we need for XMPP, especially given that we have compression methods both as part of TLS and in standalone form as specified in JEP-0138: Stream Compression (for which the Last Call ends tomorrow).