If you want to use Xpresso, I'd guess that you'd need to use the Hair Points Node, in conjunction with some iteration nodes to loop though each point on each hair guide and set it's Rest State's strength at a curtain frames along the timeline. I don't know enough XPresso togged this to work properly, but it sounds like a lot of work to be honest.
But the Hair systems Brush tool does have a vertex paint option, (Options Tab, Mode Drop Down Menu) . You my be able to animate a vertex map and use the pose morph to go between the 2 states of 50% - 0% strength, as in the Pixel lab video above.
Hope this helps, I'm out of ideas.