What is the recommended way to extend class behavior, via class_eval and modules (not by inheritance) if I want to extend a class buried in a Gem from a Rails 3 app?

An example is this:

I want to add the ability to create permalinks for tags and categories (through the ActsAsTaggableOn and ActsAsCategory gems).

They have defined Tag and Category models.

I want to basically do this:

Category.class_eval do
  has_friendly_id :title
end

Tag.class_eval do
  has_friendly_id :title
end

Even if there are other ways of adding this functionality that might be specific to the gem, what is the recommended way to add behavior to classes in a Rails 3 application like this?

I have a few other gems I've created that I want to do this to, such as a Configuration model and an Asset model. I would like to be able to add create an app/models/configuration.rb model class to my app, and it would act as if I just did class_eval.

Anyways, how is this supposed to work? I can't find anything that covers this from any of the current Rails 3 blogs/docs/gists.