Wien bridge using single potentiometer
Are you forced to use the WIEN oscillator (named after Max WIEN) topology ?
There are other oscillator types with single-element control - however with two operational amplifiers.

Here is a corresponding link:
http://m.eet.com/media/1143038/17995-82202di.pdf

I like to mention that for frequency tuning you only have to tune one single grounded resistor. More than that, the quality of the output signal (THD) - for most applications - will be sufficient, even without additional amplitude control mechanism. 

EDIT: To all who also are interested in the update of the referenced paper:
Try this link (I hope it works) and search for "A novel harmonic Oscillator: GIC Resonator" :
https://www.researchgate.net/profile/Lutz_Von_Wangenheim/publications