My thoughts return to the PSP with its serial inline remote for controlling media playback. Though I haven't yet driven with my N800, I can imagine myself wanting to use something tactile while keeping it docked, and not have to try to touch the right spot on the screen, so I can keep my eyes on the road.

I would imagine there are many apps that could benefit from extra hardware buttons as well.

I haven't seen any mention of such an accessory, so I'm asking you all if something like this exists, and if not, how much work it would be to create one.

can you buy a usb number pad and modify it? for maybe US$12 or GB£7 you can get something pretty hackable, e.g.
https://www.think-online.com/usb-number-pad-p-5624.html

BT remote is also possible, but anything (as 10-key pad) that shows up as a keyboard has the benefits and disadvantages of being able to remap the keys.
That's good, because it means you don't need special software, it's bad because it may interfere with using the same key (on a normal keyboard) for its normal purpose.

I agree, Benson. To be maximally useful, it would have to be something that can be easily reproduced. This would encourage more developers to implement it into their applications. The keyboard/number pad idea seems to me like it would be the simplest option, since the devices exist and are already integrated with the system to an extent. Could an application pay attention to which peripheral the key input comes from?

ideally what we need is the avrcp BT profile to work with canola , kagu , etc.... that would be the best solution

ftdi usb parallel port thing???