Couldn’t you just pack POM maps into the R, G, B, and A channels and then use Lerp “masks” based on whatever differentiating rules you wanted to control what POM goes where (i.e. angles/heights/surface types) to control which Heightmap Channel is referenced?
I.E. use a Make Float 4 node and have lerp masks determining the 1/0 values for POM with the texture-packed POM T2d channels as references to essentially make a custom POM Heightmap.