It's kind of both, since if the programs can't coexist it won't matter if sound can be routed from one to another. Luckily, Q Radio solves both issues.

Low and behold, Qradio does it on N900. Start the radio, tune your station. Then start the stock camera and set video mode. Sometimes I need to wait a little while till all the camera buttons get rendered onscreen. Then hit record and go.

Oh, and it doesn't seem to matter if Q radio is playing to speakers or headphone. Camera records sound either way. Just can't listen along if sent to headphones that are merely a short patch cable to nowhere.

I haven't tried qml radio on N900 yet. I did try qml radio on N9 and it always quits playing when I try to record video.