Photopea was able to open and edit the raw. Though when I imported the new height map into the map editor, it set any lines to 256 height and everything else to 0. Don't know if I did something wrong. I'm rebuilding the map manually, but would be nice to know how to generate height maps outside the editor, as it can be very frustrating to do certain things.
Edit:
Photopea works! When opening and saving a height map, make sure to set Byte Order to "34-12" or it will be completely messed up when you import it back into the map editor.