It turns out there is a subtlety in the syntax for HyperTerminal 5.1:
>hypertrm /t 127.0.0.1:21
Will open a HyperTerminal session on address 127.0.0.1, port 21
>hypertrm /t127.0.0.1:21
Will try and open 127.0.0.1.ht and complain when it is not present.