Changed GrowlMail's Installer package to install to a subdirectory of /tmp, and then move these files to the correct place within the user's Home folder, as suggested by Jonathan “Wolf” Rentzsch on Twitter.
This sucks, but it appears to be the only way to overcome x-radar://problem/7215243, where Installer installs to the base of the volume that the user's Home folder is on instead of to the Home folder itself.
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