As someone who oversees the work of writers, copy editors and designers, there’s a difference between writing and design that would make a the concept of a freelance, remote design editor problematic.
Copy editing is mostly divided between checking for errors and clarity. The rules of grammar, usage, punctuation, style, etc., are consistent. If there’s a question, there’s a style guide with the answer. Things like clarity, brevity and tone are a bit more fluid, but even there, it’s reasonably obvious to a good copy editor when something can be written better.
Even in a freelance situation, those rules stay just about the same, and when they differ, they differ consistently from one country or style book to the next.
Design is different in the sense that it’s more subjective. Changes would entail more problematic procedures and consequences than adding a semicolon or tossing a period into a run-on sentence in a Word document.
Design files are also often printer-specific. One printer might require an eighth-inch bleed, while another might ask for a quarter inch. Various settings, like dot gain, need to be adjusted for different stocks and presses, but the editor wouldn’t have this kind of information unless the designer communicated it. Of course, the designer probably wouldn’t need the help of the design editor if she knew enough to tell the editor to check for it. Fonts would need to be sent, which gets into licensing issues. Software version compatibility would also be an issue.
A bigger problem, still, would be the liability issues of a botched print job. When a freelance copy editor sends back edited copy, the client is expected to read through it, then sign a statement acknowledging that all changes meet with the writer’s or publisher’s approval. This would be problematic for a designer checking the work of the design editor, since the designer wouldn’t know what to check.
An experienced designer wouldn’t need this kind of design service anyway. There are only a few dozen things to check for to make sure files are set up correctly, and most good, experienced designers know those things in an out. With writing, however, there are millions of potential problems, and even the best writers need a second and third look through their work.
I don’t think the concept of a freelance design editor/proofer is out of the question, but it comes with more problems and gotchas than editing a few pages of copy.