We have discussed different scenarios where items link to other items in different projects but have not as of yet determined a good solution. The questions that come up are how much of the original item controls the linked items. For example in your scenario the original item may maintain the test case description but the sub items can change the assigned to and release. If I understand correctly you're primary goal is to not have to recreate the test cases description for each sub project. You could for example create relationships downstream to the sub project test cases. Then use the suspect flag to alert users in the sub project that a test case has changed. Then you would only need to copy the test case the first time and create a link.

Hopefully this helps. The link question does come up but we are still seeking the best solution and for today the relationships can provide a good alternative.