I would try a Nokia N900 ((not) as Bluetooth keyboard). But i never had a n810, so hopefully more knowledgeable people may assist.

Same company only one generation difference. So hardwarewise it should be more tested than other 3rd party devices.

Software on n900
In theory run
https://wiki.maemo.org/Bluemaemo on N900 and connect to n810

In reality i would be surprised if you get it to run, cause you will hit a lot of bugs.

But if you cant get BlueMaemo to work you can at least switch to a more "modern" portable linux device with hw keyboard

Good News according to
http://talk.maemo.com/showthread.php...197#post236197
N810 to N810 worked in 2008.

but according to
https://wiki.maemo.org/How_to_connec...tooth_keyboard
it's also "not difficult to find a compatible model" ..