Configuring baud of USB-Serial bridge
It's funny how writing a question helps one think through a problem more thoroughly. It turns out there were two issues:


Although I'd seen this link before, I realised I had still made the same mistake; I'd tried a much lower baud, rather than much higher baud, than the bridges support, so it would have disregarded my command like in the link. However, changing the baud rates through device manager to sensible (but different) values, they still communicated as if on the same baud rate.
What finally got it to work, was not setting the baud through device manager, but through hyperterminal (or putty as I'm using). It's always the obvious things!