By varying power factor and magnitude, I mean that I'm trying to develop an AVR which can appropriately react to changes in load from leading to lagging. For example, if a lagging load is being supplied an additional degree of excitation current would have to be supplied when compared to a unity load of equivalent magnitude to maintain the stator voltage. However if a leading load were to be switched in/on this would cause a voltage rise at the stator terminals, requiring a fast but appropriate reduction in excitation current to prevent damage to the load due to over voltage.
I don't intend to dynamically shape the AC output waveform, I would like to sample the output as rapidly as possible and adjust the rotor current appropriately (also in a rapid fashion). Whether this takes the form of zero-cross with a delay, or utilising some other method, is unknown at the moment.
Looking into car alternator packs and analogue circuits as you suggested, it appears I may be able to utilise the Uno to trigger the gate of a FET-type switching device. However I still need a way for the Uno to read/sample the output voltage and act accordingly.