OK, so here's what I came up with for now:
a "rescue pack" which enables an SSH server in diags, plus a simple startup script that checks for the existence of a /mnt/us/ENABLE_DIAGS file, and reboots into diags if it is present.
It works on my device (and should work on others), but please report back. And make sure to read the README.txt
PS: Yes, I'm fully aware that this can't solve all of the "brick situations" (it requires the main and user partitions to at least be in a "somewhat" bootable state), and that USB downloader mode/fastboot etc. would be much preferrable, but... well, until somebody figures out how to get there, I guess that this is better than nothing.
PPS: I didn't bother to include the sources; they can be easily obtained using "kindletool extract".
UPDATE (2013-03-13): Fixed a serious issue where SSH access would only work once on certain models. If you currently have installed a previous version, I STRONGLY recommend to download and install the current one.
Package updated to support all currently known Paperwhite and Touch models (10 in total).