While it's not exactly "easy" and probably not what you're looking for - I've been doing exactly what you want to do on my N810 almost every day for months.

I use KDE as my desktop environment (instead of Maemo) and in KDE you can map out hotkeys (aka: Input Actions in KDE Control Center) for pretty much anything you can imagine. I've mapped out hotkeys for Next, Previous, Volume +/- and Play/Pause commands in XMMS. That way while I'm reading in FBReader or KPDF or surfing the net - and I want to control the music in the background I just tap a couple of keys and voila!

BTW, when I say that it's not exactly easy - I mean that installing KDE and setting everything up can take a bit of time and effort but IMHO, it's more than worth it.