What do you mean by throw away? The submitter can always make the requested changes. And if the pull requests are not even relevant then it's entirely a different issue.
Meaning the submitter just spent one week writing code that will have to be written over again. If they had requested feedback informally before the PR, they would have written the code correctly the first time.
Having a PR workflow in place doesn't (or at least shouldn't) stop that person from seeking out that informal feedback earlier. To me it seems like what you're really looking for is pair programming, not pre-completion reviews.