The issue might be that the file is unzipped - and might not be related to the ROM.
Usually, .ipk files are gzipped tar files, i.e. created by "tar czf package_version_arm.ipk". Note that the _arm is also required.
Downloading these files on a Mac might trigger the Mac builtin unzipper, i.e. it becomes a plain tar file.
Try the following on the Zaurus:
1) "tar tvf package.ipk"
2) "tar tvzf package.ipk"
One of both must fail. The other one should show "control.tar.gz", a "data.tar.gz" files.
If 1) succeeds and 2) fails, simply gzip the file and rename the resulting .ipk.gz file back to .ipk.
-- hns