I've been wondering too about self-oscillation of the state variable filter. I did exactly what cdh suggests, remove the limitation in the resonance setter.
With a resonance value of about 50000 the filter self-oscillates and produce a very nice sine wave, with pitch changes following cutoff frequency.
For the 5.0 limit, I believe it's an arbitrary level. The sound really changes between 0.5 and 5. If you go beyond 7, there's almost no audible difference. The more you increase, the less it makes a difference. And then at some point it starts oscillating.
I believe using some exponential scalling you permit both a usefull range and the ability to self oscillate, but I failed to find something working. I know some vintage analogue synth use an exponential potentiometer for resonance control.