Check to see that you have the "HFSplus
" kernel module loaded. If the HFS+ filesystem is non-journaled, you should be able to mount-up HFS+ filesystems read-write that way. However, as that's an "EXPERIMENTAL" module, I wouldn't explicitly trust it.
Depending on how you're backing these up to tape, there might be a better way to change the permissions. For instance, tar has a few options you might find interesting;
Interesting Options from GNU Tar's ManPage
do not attempt to restore ownership when extracting; -o exhibits this
behavior when extracting an archive
this option causes tar not to store each file's extended attributes
in the archive and not to extract any extended attributes in an archive.
This option also enables --no-acls and --no-selinux if they haven't
been set already.
change owner of extraced files to USER