I was just playing around with this myself.
I think you can achieve what you want in a round about way.
Rather than using a single traditional Splat map you need to first extract each of the channels of your splat map into separate 8bit greyscale png files.
Then follow this tutorial to import the 8bit splats into each of your landscape layers: Weightmaps
From there you should be able to paint over the splats (weightmaps) the normal way.
Hope that helps.