attached my version to this post. as far as i can tell it works ok.
basically the same as cyrille's idea..
input phasor~ is fed into a one sample delay [z~ 1]. then the difference between the delayed value and the current value is multiplied by samplerate. if the absolute difference between values is more than 0.5, then the signal is zeroed, to avoid clicks on the change of the original phasor.
not vanilla, but requires only zexy.
also, you can drive not only phasors, but any other oscillators with this too.