I believe it is a good idea in principle. It could be folder’s option (not the global one) and, of course, should be disabled by default. Another counter to full rehashing could be added to the interface together with some radio-button to select behavior: 1. As when changed regularly; 2. Conflict; 3. Replace by remote copy.
Another option that I would like to see is access to the hash value through REST API. And, potentially, to each piece of a big file to control partial downloading. For example, I have a huge zip archive and what to extract few files. So, I could sync only the header and the part which contain the data.
It seems such options do not contradict the general concept of Syncthing.