You can use Powerlaunch to control all sorts of things via the N8x0 hardware buttons.

Maybe what you want to do is also possible with powerlaunch. There is a similar usecase mentioned on the powerlaunch homepage:
"I use powerlaunch for a pocket music mode where I press a set of hardware keys and it turns off and locks the screen only. The keys can then be used to control music/volume while the device is in my pocket. This also works great for playing music while driving without requiring looking at the device."
http://powerlaunch.garage.maemo.org/

Unfortunately Powerlaunch is a little bit difficult to configure. So I have no idea how this can be done.