Does anyone know of a way to have a character "paint" biomes?

Im working on a nature simulation game and one of the things we would love to have is the ability to paint biomes as part of the game? So for example the player could maybe click on the landscape and an area around the click point under the mouse on the plane would turn that selected texture (for example grass) and that would visually represent the grass biome. Then the other part of the ability would be some way to see the current biome if you right click? Or if biomes are overlapping then what percentage of each biome is at that click point?

We cant think of a single way of acheiving this and it feels like the final piece of the puzzle for us! If anyone knows a way we would greatly appreciate it!

not sure if I quite understand your mechanic, but probably use render targets.