Just to wrap that up, I finally got home today, back to my USB cable, and indeed it works. In case anyone wonders what I did:

First, put the device into RD mode, disable the watchdog, then reboot:

Then wait for the device to come up and be visible on the network. The display is just plain white, but the tablet will connect to the wireless nevertheless.

Log in, and as root do an "apt-get update" followed by "apt-get upgrade" -- this installs a new rtcom-beta-os2008 where the problem has obviously been corrected.

Reboot again and (after checking the N810 now boots correctly!), put it back into normal mode

Phew! No need to reflash and reinstall everything!