We know this is not documented anywhere, but here’s why this is happening:

For Components to be available in a Popup, they must be createable from JavaScript — e.g. the component must not require any Apex / server-side logic to be run, it must be able to be created on demand from JavaScript.

If this is true of your custom component, then you just need to add the following property to your Builder definition:

 isJSCreateable: true

So for example you just need to add it to your Builder definition like this:

skuid.builder.registerBuilder(new skuid.builder.Builder({
   id: ‘practifichatter’,
   description: ‘custom chatter’,
   isJSCreateable: true,
   
etc. etc.

Thanks Zach… works like a charm.