Additive Inheriting of workflow approval groups

I want to set up approval groups where a certain set of individuals have approval across a whole product line while other groups have approval only for specific product workspaces within that line. I've just discovered that inheritance for workflow approval groups is half-baked, so that as soon as you add a specific member at the product level, you lose all the members inherited from the product line. A checkbox to allow cascading of approval group membership would make management of multiple approval groups so much easier in the absence of any other effective user grouping feature.