If I remember correctly, there was some discussion around R2 acting as a proxy in front of s3. So, if R2 does not yet have the asset, you would automatically pull it in from S3, and from then on R2 would serve it directly.
Cache reserve is a similar concept but a bit of a different product (although some people might be happy with the no-code approach that enables). Migration is a bit of a different thing.
Is that still on the cards?