I need to modify the Product Details Page Layout, which it looks like I could do by defining a new template in an extension. Does anyone know if this extension needs to be defined as a CCT to work? It's deployed and activated but not showing up in the SMT interface as an alternate. Thanks!

I think you would want to use a pagetype for a different layout

You do define the new template in an extension, but it is not a CCT. You use PageType.registerTemplate().