Painting materials with out a mask

Hello all,
I am making terrain with procedural textures applied by one material and wish to hand paint another material onto the mesh. Looking online the only solution to do this I found is use a black and white mask texture and mix RGB node, however due to the sear size of the terrain I have the UV of the mesh mapped so textures are repeating so this solution of a mask can not be used. Are there any other ways to go about this?

Pic related is the scene witch I wont to apply a forest texture under the trees placed via particle system. I also want to paint other materials/textures in the same manner like farmland.