Phaser Spread is among the parameters which are missing from the Mod Matrix and LFO Mod Destinations. Interestingly, it's the only Phaser parameter that is missing. Feedback, Frequency, Mix, Mod Rate and Mod Depth are available.
Since, depending on the settings, the Virus Phaser can be used as several one pole or notch filters, modulating the Spread would allow for nice (or rather evil) sounding formant-like effects. Thus it would be a most welcome Mod Destination.
However, Phaser Spread is available as a destination for Soft Knobs, thus I guess it could easily be modulated as well.