OH YEH
It's alive and booting (root, not kernel) from an old pogoplug's usb drive. Now time to figure out how I managed to do it, and perhaps prod that watchdog a bit. Woof!
Edit: well, that was fast. Figured out the watchdog in 2 minutes... or at least, I got it to activate and reboot the system. Time to see how to deactivate it!
Edit #2: Awesome. A kernel panic at shutdown!
$this->bbcode_second_pass_code('', '...
[ OK ] Reached target Shutdown.
Sending SIGTERM to remaining processes...
[ 52.795948] systemd-journald[76]: Received SIGTERM
Sending SIGKILL to remaining processes...
Unmounting file systems.
Unmounted /dev/mqueue.
Unmounted /sys/kernel/debug.
[ 52.999191] EXT4-fs (sda2): re-mounted. Opts: (null)
Disabling swaps.
Detaching loop devices.
Detaching DM devices.
[ 53.085010] sd 0:0:0:0: [sda] Synchronizing SCSI cache
[ 53.126707] sd 0:0:0:0: [sda] Stopping disk
[ 53.226838] Power down.
[ 53.231469] Kernel panic - not syncing: Attempted to kill init!
[ 53.237466] [<c000d290>] (unwind_backtrace+0x0/0xe0) from [<c03c6d14>] (panic+0x58/0x198)
[ 53.245687] [<c03c6d14>] (panic+0x58/0x198) from [<c0021a48>] (do_exit+0x88/0x7a0)
[ 53.253317] [<c0021a48>] (do_exit+0x88/0x7a0) from [<c0032e40>] (sys_reboot+0x118/0x1d8)
[ 53.261459] [<c0032e40>] (sys_reboot+0x118/0x1d8) from [<c0008b40>] (ret_fast_syscall+0x0/0x2c)')
Edit #3: Ack! Wrong watchdog. It appears there is another meaner one.
Edit #4: I fail at watchdogs. Or I just give up too easily.
Edit #5: I'm going to try something really stupid: have the system boot stock kernel, then reboot into archlinuxarm kernel. This is how I'm going to attempt it: When system boots, check whether kernel is stock or archlinux. If stock, copy archlinux kernel into uImage and reboot, otherwise, copy stock kernel into uImage. Let's see if this will blow up on me...
Edit #6: Apparently, I fail at kernels too. How do I extract a kernel from an mtd device?