After posting to ask for help with this technique, I went out and bought some W&N professional watercolor to use instead of the cheap stuff I already had. I'm pretty happy with the result!
For comparison, here is the first version. This was done with cheap student grade watercolor that I already owned. I didn't have masking fluid, so I painted the trees and highlights with white watercolor.
But this paint didn't work once I got masking fluid and tried to re-wet the paper. All the dried paint reactivated and made a big muddy mess of the painting.
Here is the finished piece using the W&N paints, masking fluid, and a white gel pen for highlights.