Kobo Aura H20 (1st ed.) bootloop

[emacsomancer]

I've managed to get my Kobo Aura H20 (1st edition) into a bootloop.

It starts into the cycling three dots display, and flashes black, and reboots after a time.

I cannot make it turn off by holding down the power button (it just reboots instead).

Resetting with a paperclip in the pinhole has no particular additional effect.

I've tried holding down the power button for long periods of time; doing combinations of holding the power button and resetting with the paperclip in the pinhole; tapping both bottom corners while it's rebooting; and so on, and nothing seems to have an effect.

Is there anything else to try? Aside from prying it open and reflashing the internal sdcard? (which I'm assuming compromises the waterproofing)