Routing multiple Joomla/virtuemart installs to one images/stories/vm folder?

[wizardofaus]

I have a total of four VM 3 installs that are on the same server (under separate domains), and while they do not all have the same content, a lot of what they have is consistent between installs (same product and thus product image, different site setup, categories, etc.) However, this makes trying to keep the image folders in order a bit of a nightmare when items need to be copied over from install to install. I was wondering, is there a good way to get all four of these installs to grab from and store to the same folder so I don't need to keep multiple copies of the same files on the server, eating up my server space? I've experimented with this in the past and had no luck, but thought this would be a good time to check and see if anyone else has come up with a solution.