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.