This is THE moment for me to thank PenguinBait for his outstanding work!
While I wasn't successful on my first try using PB due to:
- operator error
- "missing manual"
- Had to update the bootmenu stuff to allow timered autoselection of bootpartition
the experience still was immensly joyful (took me perhaps 30 minutes to finally get it right):
a) I felt somehow "in control" on this machine again (aka Master of Disaster).
b) Having cloned to a 1 GB partition of the internal flash gives a LOT of space to install additional apps and makes the N810 so much more useful.

Lessons learned:
- Not any partitioning works (for me to get it working, I had to make partitions on the internal flash mmcblk0, the first one a W95 FAT32 (LBA), the second one a Linux ext3)
- Take care to really select an option... (press SPACE)

The "missing manual" was probably the worst part. In between outdated posts here and old wiki articles, the currently relevant information got lost (at least for me). Having a proper doc on a homepage somewhere would have helped a lot (perhaps it even exists but I didn't stumble over it) - especially as the "concepts" are probably murky for many of us who don't have an extended linux background.

Thanks again, pb!