Please see http://www.virtualbox.org/manual/ch04.html#sf_mount_auto Please add the group vboxsf in the ebuild and create a note like elog "Please add users to the \"vboxsf\" group so they can" elog "have share folders automatically mounted"
There is already a "vboxguest" group. I would guess that substituting it in getgrnam("vboxsf") in src/VBox/Additions/common/VBoxService/VBoxServiceAutoMount.cpp should do the trick.
Please do not modify the sources but implement the fix by adding the group and a /media directory - every documentation out there (and probably quite some tools and scripts) uses these defaults and it is extremely confusing and frustrating to search for the reason why your own configuration fails.
The oldest version in the tree is 4.2.24. Is this still relevant?