To write a telnet chat app in node.js would require you to use raw sockets. (http://nodejs.org/api/net.html#net_class_net_socket)
Http runs over TCP (sockets).
Can you do both? Well, yes, but not exactly easily. The issue is both http and telnet would create the socket connection, but the telnet client wouldn't do anything else until told to, whereas http would send the http request.
You could use the lack of the request for some period of time to be an indicator of telnet client, but the second issue is that it means you have to manually receive and reply for the http requests.