The `unpack` function used in ebuilds to unpack source packages supports a lot of compression formats, but lzip is not one of them, although the `tar` executable has built-in lzip support (through the --lzip option). lzip being designed for long-term archiving, it makes it perfectly suitable for package archiving and distribution, so I think portage should support it. Moreover, portage binary package support lzip compression.
*** This bug has been marked as a duplicate of bug 249059 ***