Copy just permissions and ownership from one tar file to another

I'm not sure Tar can do that. It should be possible, though, to untar both backups (to different directories), then use something along the lines of

cd /mnt/oldbackup ; 
find ./ -exec getfacl {} | setfacl /mnt/newbackup/{} 

as the output of getfacl can be used as stdin for setfacl according to the manpage.

There is no way to put new meta data into a tar archive, but there is a way to extract just the meta data from a tar archive to the filesystem. Just call:

star -xp -meta < archive.tar

This will not create new files or overwrite file content but just restore the meta data from the archive to the files in the local filesystem. You most likely need -U in addition (see star man page) for "do it even though the file in the archive is not newer than the file on the filesystem".