Boyster: I'm not sure so that it would be such a headache to implement. If you try the cl_pitchup thing posted earlier, you can almost do a vetical 360 degrees. If they could give it a nudge so that you didn't stop turning at a certain point, it'd be great, despite a little grapical weirdness. But saying "it's impossible to code" isn't really a fair point. Ultimately, that's up to the team to determine, not you or me.
Experimenting with the cl_pitch thing has convinced me that a more fleshed out version would be cool. While I was testing it against bots, I flipped backwards in midair and fired a SBC at someone swooping at me from behind. Very cool stuff.
There could be an option that turns 360 flying on and off, so if you don't like it, you don't have to use it.