I thought it might be interesting to periodically measure the speed of a player's connection and show their "ping" time, in the "who" output for example.
It looks like measuring the time it takes to receive a response to an "are-you-there" in the telnet protocol is the most analogous mechanism to ICMP echo, but I'm thinking only standard telnet clients are likely to respond.
I suppose one could just repeat the telnet linemode request, which is probably close to being universally supported across clients, and measure the response time to that.
You could always pick a nonexistent telnet opcode and send an IAC+DONT+OPCODE message. Unless you stumble across someone's custom telnet option, almost any telnet client is going to send back IAC+WONT+OPCODE.