Can you have a Socket.IO (express) server be loaded from a web browser?
That's explicitly not possible due to the design of WebSockets. It starts as a special HTTP request that, after the handshaking, drops the HTTP protocol and strips it down into the WebSocket protocol -- a nearly bare protocol similar to (but slightly more managed than) raw TCP. Because a web browser specifically cannot handle HTTP requests, it could never initiate the socket as a server.
This was done specifically so it wouldn't be possible to write a drive-by botnet website to use scores of users' computers for DDOS attacks without their knowing, amongst other security concerns.
So it wouldn't surprise me if Flash supported that kind of behavior. ;) (I know Java can, but who enables Java applets?)