Something like that looks reasonable. One major comment, though, is that the output of tar should be parsable, i.e., it should be possible to look at the output, figure out what format it is, and parse it reliably. The current format doesn't have that property, but these extended formats ought to, no?
A nit: in your proposal the tar -tvvvf output didn't have the trailing "." or "+" that the tar -tvvf output did -- was that a typo, or if not then what's going on?
