This is a straight recompile from the Ubuntu source. I've done a bare minimum of testing; it can unpack archives on the device, and create archives that then unpack fine on a Linux box.

There are two versions, as provided by debian/ubuntu, p7zip (1MB installed) and p7zip-full (3.8MB installed). In the "p7zip" version, there are two command-line executables, 7zr, and a wrapper script, p7zip. The "full" version can also unpack zip, tar.gz, bzip archives, and other formats. It provides the 7za and 7z executables, plus the libraries for other formats.

If you just need the unpack .7z archives, the p7zip version is adequate.

Download the packages here:
p7zip
p7zip-full