Inverting effect of resistance in an RC circuit
You could consider using a 555 timer circuit that uses a constant current source to charge the voltage up on the charge capacitor. The constant current source requires normally 3 resistors to set the current and one of these resistors, when increased will linearly (to a point) create greater current as its value increases. A greater constant current (higher resistance) means the capacitor charges more quickly hence, F is proportional to R.


(source: next.gr)  

Note that increasing R1 causes the charging current to be greater thus increasing the speed at which C1 charges and hence frequency rises. Circuit taken from here