I got bored and wrote this component to support Unix Tape Archives (.TAR), gzip files (.GZ), and bzip2 files (.BZ2).
You can download it here (http://www.foobar2000.org/components/view/foo_unpack_unix).
I must give some warning for large archives. If you cram a 256MB TAR into another archive, such as gzip or bzip2, it will have to be unpacked to memory before anything can be extracted from it. Such is the design of the archive support services.
Oh, and I didn't really do this because it's in any way more efficient than archive formats already supported. For example, the Tenchi Souzou (Terranigma) SPC set, a format that typically benefits from solid compression:
146,367 ts.7z
164,313 ts.rar
467,025 ts.tar.bz2
2,014,631 ts.zip
2,039,682 ts.tar.gz
As you can see, gzip solid compression doesn't really benefit at all over ZIP's single file compression. And while bzip2 outperforms both, it is trounced by both RAR and 7-Zip.
So, it's not terribly useful as a replacement for formats supported by foo_unpack and foo_unpack_7z, but it could be useful for reading downloaded archives of 100% legal to distribute music while you're still sharing them. Assuming you don't just unpack, retag, and rename to your own tastes, like I do.