Ok, so I just went suicidal and modified the file in the wrong way. ("jbpatcher.jar" instead of "jbpatch.jar").
And yes, my device is now bricked.
And here is how I unbricked it:
For a short amount of time (10 secs or so), the Kindle still shows up as a USB device, so I issued "touch /media/Kindle/ENABLE_DIAGS" in that timeframe. After two more reboots, I ended up in (SSH-enabled) diags. I ssh'ed in, and:
mount /dev/mmcblk0p1 /mnt/mmc/
idme -d --bootmode main
And all is good again. I'll update the first post with a big fat warning concerning the init.xargs editing