Yury Delendik on the Shumway team is working on RTMP.js. It's a standalone JS library, but will eventually be used by Shumway.

https://github.com/yurydelendik/rtmp.js

AFAIK, Shumway does not support UDP sockets, but, as a Firefox extension, it could access Gecko XPCOM APIs that are not available to JS on the web.