You can't extend or inherit CatalogContent. A workaround is to have an immediate Node, like Shoes, under Shoe Catalog, which is an extended NodeContent class with proper AvailableContentTypes attribute. 

Does this mean you create your catalog root (Type:Catalog) and then extend the Category node with proper AvailableContentTypes. Or are you talking about a completely new type of node in addition to Category, Packages etc

It is like this 

Shoe Catalog => Shoes => Brands => .... (shoes)

You can have ShoesNodeContent extends NodeContent and add AvailableContentTypes to it. Then create Shoes from ShoesNodeContent 

Something as simple as? - 

    [ContentType(
        GUID = "FFE372D9-EBEA-48C9-B723-3293E1B2A53C",
        DisplayName = "Shoe Node",
        Description = "Shoe Node")]
    public class ShoeNodeContent : NodeContent
    {
    }

This causes "Multiple custom attributes of the same type found"

What are you talking about? This is from our QuickSilver:

    [CatalogContentType(
        GUID = "a23da2a1-7843-4828-9322-c63e28059f6a",
        MetaClassName = "FashionNode",
        DisplayName = "Fashion Node",
        Description = "Display fashion products.")]
    [AvailableContentTypes(Include = new[] 
    {
        typeof(FashionProduct),
        typeof(FashionPackage),
        typeof(FashionBundle),
        typeof(FashionVariant),
        typeof(NodeContent)
    })]
    public class FashionNode : NodeContent
    {

    }