I did find a solution not long after asking, but later I no longer needed it and it has been two versions back since I last used it. Unfortunately, I no longer have the project and I cannot remember how I solved it. What I do remember is that it did involve creating a second RT just for the shader, executing the shader, and then copy the data from that RT to the actual RT used in the material (either created at runtime in code or Blueprint, or as an asset).