I can now stream 720x576 UYVY frames at 25 fps from the unit and pipe the output into mplayer, which can play them.
Obligatory screenshot follows (which doesn't do justice to the fact that it's motion video...but it would seem unfair of Carole Hersee to miss out, now she isn't cut in two!)
|One full frame of test card F|
I've posted the source code that you need to do this to a public github project easycap-somagic-linux . The code is incredibly rough yet, and there's plenty of to-dos on the list, but they will have to wait until I have more time. It's usable as far as I need it to be, for now.
I'm hoping that someone else will pick it up and turn it into a proper device and video4linux driver.