I'm attempting to figure out of there is a way to assign a button on the H9 stompbox to turn off the delay function in the H910/H949's #3 preset Women In Love for Van Halen's "Women In Love" song. The delay is only present in the harmonic tapping segment of the intro and then the delay is turned off and the rest of the intro is just straight picked arpeggios with no eighth note delay following each note.

Well, if you really want to do it with an H9 footswitch, you could create two different presets — one with delay and one without delay — and change from one preset to another, but there is a bit of a lag when changing presets.  And this might not work for you if this is supposed to happen in the middle of one part.

Another way to go about it would be to use an expression pedal.  You could assign the bit with delay to the heel expression pedal position and the part without delay to the toe expression pedal position (or vice versa).  If you're interested in going that route, there are tutorial videos on youtube about creating H9 expression pedal patches, and this is also covered in the H9 Control manual on pg 14 (https://s3.amazonaws.com/com.eventide.downloads/Product+Manuals/H9ControlManual.pdf).