How to use ‘tar -c’ without any optional arguments

You want to use the --to-command option of tar:

tar xf tarfile.tar --to-command='sh -c "mkdir -p $(dirname $TAR_FILENAME) && base64 -d > $TAR_FILENAME"'

This is described in the manual here.

backup-specs is part of the backup system the manual claims is distributed with tar, however my system lacks this and I don't think actually uses it, at least not anymore.

You could mount the archive with archivemount or mountavfs and recreate it again

archivemount tarfile.tar /mnt
cd /mnt
tar cf /tmp/tarfile.tar --transform 's/foo/bar/' .

write operations on the archive filesystem will perfom a full rewrite on umount, so don't seem a good option for large files.

EDIT

I don't know implementation details but seem like we are saving the write files to filesystem step.

Just test to solve dudes, (over a tar of my /usr)

#!/bin/bash

# try to avoid slab cache issues 
cat /tmp/usr.tar > /dev/null

T="$(date +%s)"
tar xf /tmp/usr.tar
tar cf usr.tar usr --transform 's/usr/foo/'
T="$(($(date +%s)-T))"
echo "Tar/Untar seconds: ${T}"

T="$(date +%s)"
archivemount -o readonly -o nobackup /tmp/usr.tar /mnt
tar cf usr.tar /mnt  --transform 's/usr/foo/'
umount /mnt
T="$(($(date +%s)-T))"
echo "Archivemount seconds: ${T}"

T="$(date +%s)"
mountavfs
cd '/root/.avfs/tmp/usr.tar#'
tar cf /tmp/test/usr.tar   --transform 's/usr/foo/' .
T="$(($(date +%s)-T))"
echo "Avfs seconds: ${T}"

Output:

Tar/Untar seconds: 480
Archivemount seconds:  failure, a lot of read errors.
Avfs seconds: 217

So Avfs wins!.