I can't resist to post my progress.

Once again I tried to find modern replacement for the n810, but my three hard requirements (must run emacs, must have a hw keyboard, must run a week when idle) could not be satisfied. So I bought a used n810 on ebay for the price of two mugen batteries. Sometimes it is good that the n810 is a niche product.

I cloned my internal memory card's boot partition to the new device, using tar called from ssh from my desktop computer, so the big tar file would end up on my desktop machine. Then I did the inverse operation on the new device, re-installed bootmenu and the Diablo Turbo kernel, eh voila, the device looks and feels exactly like the old one. Even my debian chroot worked, once I inserted the external card. That was pretty painless.

I don't have any hard numbers concerning the battery life yet, things look promising though. ASUI reports 0.5% drain in the last hour, which looks fine to mee. Also ASUI allows switching USB from host to OTG and back, which was not possible on my old device. So I really suspect a hardware issue somewhere around the USB circuitry.

Thanks to all for trying to help.