Delicious has a great 3 part tutorial on making a pitch shifter - I think you could do it in GEN - the objects are all very low level. I am not sure if this is what you want or if it has any impact on latency but maybe it gives you a start.
Changing the window size without clicks requires a kind of sample-hold thing, where the values are updated only at the zero point of the window phase. All kinds of other things can be changed at this point too - here's a couple of modifications to change a) the delay time (i.e. pitch variance) and b) phase offset according to a stochastic 'blur' parameter: