cross project symbolic links

We have a set of libraries that need to be incorporated into different
projects but each project needs to be able to also modify the source
of the library (and possibily when the project is done or at some
other point integrated into the "parent" library)... for example our
unit testing library is in the "fnre.test" package in most cases it
needs no modification but in a project I am working on right now I
need to modify one of the classes... the three solutions we have come
for solving this in a generic way (it comes up time and time again)
are:

1. Generate a jar and place it in a common location and if you need to
modify it do so in the parent project (this seems to be asking for it
as far as best practices go)

2. Copy the parent's baseline into each project with aedist, but this
makes it hard to keep track of what projects have made what changes
and hard to control reintegration back into the parent project when
needed

3. Make symbolic links between the parents baseline and the child baseline

Note we *CAN NOT* use branching/subprojects from the parent project
for legal reasons