Possible equivalent of --mt-file when recompressing files contained zip and gzip archives

[fdelapena]

More like a question, I am not sure if files in zip or gzip archives are always separated deflate blocks where it could be compressed without less efficient deflate block split, otherwise it could be a nice performance gain for these cases.

[T-3B]

ZIP files always store separated Deflate blocks (one block for each "embedded" file).
On the other hand, GZIP is just one single block.

Having --mt-file apply parallel optimization of Deflate blocks in ZIP files would be really nice to see, indeed :)