Well, it's done - if anyone else uses the free online calendar 30 Boxes (
http://30boxes.com), then they may find this of interest:
http://www.kludge.co.uk/30RockBoxes/
It's a java Webstart application that connects to your 30 Boxes calendar, downloads it, converts it into a simpler format, then saves it onto the player for use with RockCalendar.
Admittedly the overlap of 30 Boxes users and RockCalendar users might be small, but I'm one, and if there are more - well, here you go!
Caveats:
Bear in mind that this isn't a sync app. It's purely one-way. The 30 Boxes entries overwrite the existing RockCalendar file. However, that seems fine to me (entering events in RockCalendar is a bit fiddly for me - I don't intend any criticism, because I think RockCalendar is great - it's just that there's only so much you can do with the buttons on the front of the H340).