I went and created forum topics for how to modify the TerrainlinX Planner with new markers or tiles. Hopefully others find the information useful. I was asked to also submit the modified markers as a kitbash, so that's just what I did. Here's the snapshot I provided with it:
It works pretty well, the markers. I hope to make markers for the trees from the Hinterland Forest set. That'll be pretty neat to see, I think.