Earlier quoted context omitted.
But then you are not using tar, you are doing your own file format atop of tar.
I suppose you are right about that. But it would still be a valid tar file that can be viewed and extracted with normal tools. Kind of similar to how a .docx file can be extracted as zip but still has additional structure to its contents.
What are you really proposing? That a first ".INDEX" entry be made that contains the offsets of all the other members?
That could work in a backwards-compatible way (as long as no standard tar utility makes modifications to the archive...), but it's hamfisted. Just use Zip. It's already a well-known format with numerous implementations and already does the job that you want to do.