Writing render targets to disk while isolating channels to separate files?

I am writing values from my render target to disk for use as splat maps for landscape layers. The problem I have run into is that landscape layer info has pretty specific requirements for files used for import. I need to be single channel 16 bit png that matches the landscape pixel dimensions. I can get most of the way there with export render target or export to disk. Both work fine so long as they export a RGBA… but I need just the R. When I try and bring that RGBA back in I get a corruption warning instead of a nice map. The render target can handle being everything I need but when it exports things drop to zero bytes and empty. What other export to disk options are available?