When I first scratched the itch of calculating checksums for every file in a tar archive, this was my original intention. When I decided I wanted the script in bash for simplicity, I forfeited the idea and settled for extracting the files and then going over all the files to calculate their checksum values.
So when Jon Flowers asked in the comments on the original tarsum post about the possibility of getting the checksums of files in the tar file without extracting the whole archive, I decided to re-tackle the problem.
tarsum-0.2 – A read-only version of tarsum