I've found a solution that might suit you, set the rest hold to a percentage e.g 50% in the dynamics tab, then scroll down to the Modifiers section and find the Rest Hold Graph, you can animate the points on this graph to control how far along each Hair Guide the Rest Hold parameter has an effect.
Check the Hair documentation under the Dynamics section, read up on the Modifiers section, much easier than setting up the Xpresso solution.