I was testing this new feature and it didn't work for some devices as they do not support the Telnet Environment option (e.g. xinetd's telnet server) so the username is not passed to the server. I believe that there should be a regex option for the username parameter that works the same way as the password-regex parameter as that would allow to automate username input on these servers.
I made a patch that implements the username-regex option to handle the situation I described and it works for Linux xinetd telnet server. Not sure if it's the right way to implement or the best solution.