I suspect they’re already doing something similar. As they claim that writes are committed in-order. Which means, at the least, there is a timestamp associated with each write. Hypothetically, it would work similar to the cursor in the web API for long lists (listing all keys, for example). The cursor most likely contains encoded information for how much to send and what point to start at. As this preserves the “REST-i-ness” of the API.
A similar ‘cursor’ for the snapshot could basically encode “don’t send anything written after timestamp X”. So, I don’t think it’s that far out there, but I also don’t think it’s “deliver it in a week” either.
The hardest part of it is “this value is too new, so I need to send the previous version”… To answer that depends highly on what’s under the hood.