Out of curiousity, could you workaround with a symbolic link?
That's how I re-route a lot of my mac stuff to an external HD. 10.5 introduced some oddities to the process, but in theory it should work.
Just be careful with the syntax - I've done serious damage by reversing the syntax.
ln -s /Volumes/MyExtHD /Users/Shared/MyRandomLinkName
I think is how it gets done - can someone verify?