Swapping vertex color channels for Kite Demo river mesh? (need Blender import/export workaround)

I have a mesh from Kite Demo (the riverbed) that is using alpha vertex colors for a height map and red to blend between two types of rocks. Blender only seems to allow RGB vertex colors, no alpha, so the alpha channel gets lost on import/export. The mesh/material isn’t using blue and green so: is there a way to copy the alpha data into the green or blue channel before or exporting?

I couldn’t find any options in the paint tool to do it. I was thinking I could do it by swapping channels in an external paint program: I see an import TGA option, but no export TGA (not sure how that actually functions anyway though, is it based on UVs?).

I’ll give my end goal in case there is another solution that doesn’t involve this at all: the riverbed water has two layers and I am just trying to export to blender to remove one layer to lower the shader instruction count. Making one layer have an invis masked material still seems to affect the shader complexity some.

I ended up making the invis material unlit and that saved a lot on shader complexity; then I added a world position offset to push it under the riverbed and got rid of the rest. Seems good enough for now, but would still like to know if anyone knows how to do this for the future.

Can’t you paint vertex alpha on your mesh in UE4 ?

Or convert b/w vertex color brought from Blender into vertex alpha in the material ?