4th try's the charm in this case: jessie netinst (w/ USB-NIC)
Install of a minimal jessie system on btrfs root was flawless, as was the upgrade to testing. Seems like the testing version(s) of the installer don't like btrfs root, at least on this board. In any case, now everything seems to be working, except
* the onboard NIC is broken in GbE mode (being worked on)
* "kernel: /cpus/cpu@0 missing clock-frequency property" (same for @1, probably harmless)
* there's probably no video output (don't need it, haven't tried)
Stock Debian on a €45 OSHW SoC, yay! Keep up the good work!
P.S.: Should I file an installation-report? On the successful attempt (there's #780164 already) or for the failed ones? In the latter case, how do I do that?