Hmm -- looking at the picture, I had guessed that you were using that remote's 3-position slide switch. All that switch does is change the internal keyset being used by the remote control. Using the volume-rocker switch on the N900 to change keysets is pretty much the same thing.
I guess what I should really be asking, is exactly what are you looking for here: are you looking for a full-featured universal remote control that can work with many devices, or do you want to create a new customized remote control that works with just these three devices?