Rob is correct, but there's more information than just the image to transfer, and basically all of it is directly from the input file to the output file. I never imagined someone trying to replace the input images, so perhaps it needs to check for an empty pre/post-fix before attempting the process.
I'm not certain that reworking the read/write to support this use case is the best idea. At least in my processing and the other workflows I've seen, using the Subframe Selector would come before registration (and thus before any drizzle files exist). The most important reason for this is that, depending on how much the images are shifted from registration, you can massively skew the metrics it uses and improperly weight the images. Another less important reason is that you might want to use the process to determine which image you're registering to as a reference.
