I rendered out an image with a few extra image planes as a pic, then composited those planes together into RGBA channels in a COP network with channelcopy. I'm now stuck on what should be the easy part: just writing out the result to a file. I see the correct result in the composite view, which is my nice RGB image with a separate alpha in A, but when I write out a tga or png I get the alpha premultiplied and it blacks out most of my RGB.

I can get around it by outputting multiple images and comping in Photoshop, but that effectively means I can't use COPs at all for most of my purposes if I can't pack data into different channels without channel crosstalk.

I'm sure I must be doing something wrong, but what is it?

I was struggling with this exact thing for a long while as well, until I found a setting on the ROP FIle Output node, in the Metadata tab - PNG/TGA Premultiplication which you can set to Unpremultiplied