provide alter hook for content_type definition

No. :)

Which isn't to say that I think the need to tweak content type definitions is invalid. Rather, it's that I don't want to solve this problem with an alter if the goal is just to tweak category or title. The preferred solution is one where you can create one-off'd versions of the content type which are, themselves, content type instances. Because that's a step towards being able to bundle up sets of content types into packages that can then be deployed out as groups of available content types, complete with special naming and categorization.

If you can provide an argument for this beyond modifying the category/title info (that's the only use case I've been able to think of, myself), then I'd reconsider. Or, if it looks like the aforementioned bundle of work is going to get back-burnered, then I'd consider this as a stopgap. My problem with this solution is that it's a) not very portable (think Features) and a distant b) I'm trying to avoid introducing yet more alter-itis.

My specific use case would be to disable certain categories and content types for panel nodes, which I am letting low-level end-users build. For example, there are several blocks like "Who's Online" and "Who's New" that I don't even want to show to users. The bigger use case and challenge I am facing is "How can I take this amazingly powerful module and simplify it for my end users?". (ie http://drupal.org/project/simpleviews)

I understand argument a) and yes alter hooks are just not that portable (unless they are written very carefully). Alter hooks are a very good answer to the 'can I do blank?' question.

Argument b) is so reasonable ... more people should chime in.

Thanks,
~jake

You can already disable content for panel nodes. Visit admin/build/panels/settings/panel-node -- you can also control which layouts are available.

Thanks, I had no idea about this feature. It is exactly what I needed.