[jdiehl]

Personally I've never seen anything out there that would do what you are talking about. In fact I've never seen sub manufacturers in the world even. In the example given:

Dell
-Inspiron
-Latitude
-Precision

Technically those aren't sub manufacturers. Dell is the manufacturer, Inspiron and all those following are the Model. So you only have one Manufacturer. The only way you'd have a sub manufacturer is lets say something like:

Ford
-Lincoln

Since the Lincoln vehicles are technically made by Ford you might consider that a sub manufacturer, but that'd just be weird.

And I think it's because of this that X-Cart doesn't use sub manufacturers nor are there any mods. You might be able to create something like this with some indepth customization, but not sure it's worth the labor.