Hi Jim -
Unfortunately, I think this may be a feature request question. On Jive 3.0 and 4.0, I don't think there is a way to do this. This would be a great feature for Subject Matter Experts who want to be notified on just one segment (tag/category) of a Space or Social Group.
We have structured our community with specific social groups for each product. And then one space for questions that span multiple products. I don't think we've had too many issues with folks not knowing which product a question belonged to.
Sorry that I don't have better info for you. Maybe this is better handled in 4.5 or 5.0?