So after a couple of reboots and plugging and unplugging the USB drive, the router finally managed to build an index file for the drive, and (some of) the content can now be accessed by DLNA clients on the network.
Unfortunately however, as reported by others in the other thread, it does not update the index when you add or remove files from the drive, nor does it have a re-scan button.
The only way to force a rescan is to reboot the router.
As far as I am concerned, this is not an acceptable or practical solution, and it pretty much makes the built-in media server useless for all practical purposes.
This could, of course, be fixed with a firmware upgrade, but two questions come to mind: will they fix it? and if yes, when will they fix it?
Personally I am very disappointed by this. They make a big deal about this being a DLNA-compliant 'media router' – you'd think they would have tested the media server feature before sending it to production...