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 in case it doesn't.
Note: 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.
UPDATE (2015-07-05): Rebuilt to support all currently known Kindle Voyage, PW2, PW1, and Touch models.
UPDATE (2013-12-20): Updated to support all currently known Kindle PW2, PW1, and Touch models.
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.
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NOTE: If you're looking for something working on newer devices, try the one in
NiLuJe's snapshots thread. -- NiLuJe