I made a little mistake in the description above: The kernel I had on my QNAP TS-219P II before was linux-image-4.9.0-3-marvell_4.9.30-2+deb9u2 - and this appears to be the last version that boots.
I've tried loading mkimage'd kernels+dtbs via tftpd into my broken QNAP TS-219P II from these packages and _none_ of these boots: linux-image-4.11.0-0.bpo.1-marvell_4.11.6-1~bpo9+1_armel.deb linux-image-4.11.0-1-marvell_4.11.6-1_armel.deb linux-image-4.11.0-2-marvell_4.11.11-1+b1_armel.deb when I do the same procedure using the kernel+dtb from linux-image-4.9.0-3-marvell_4.9.30-2+deb9u2_armel.deb, it does boot. Not fully, since the flashed initramfs does not match the kernel version, but at least it doesn't die immediately like all the 4.11 kernel builds do.
