Ok, I've checked both of them. It is a simple workaround; rather than using the "playbin2" element, they use "pulsesink". I'm pretty sure "pulsesink" is audio-only, so I'd still have to do something else to allow for video playback.

I'll keep investigating Rockbox for the moment, as that code uses playbin2 and still managed to get it to work.

EDIT: It appears that there is a package called "libplayback" that is the official(?) method for declaring an app as being allowed to play audio during silent mode (among other things). It appears to have been created in response to bug 6694. I'll try and use it in Orecchiette...