Atomic Perception

Check out the FLIP Multi-phase example file in the content library.
https://www.sidefx.com/contentlibrary/flip-multiple-phases/ [www.sidefx.com]

Yes, just paste a complete file path to the font into the text field. If you hover over the Font label, you'll see a tooltip that reports the parameter's name as file.

Here's one way to do it. Generate a start and end frame attribute for each hair strand. Then loop through each hair and use the frame number to fit the start/end range to 0-1 for second_u.