Rails Active Storage: How to create "named variants" that are cropped by user-supplied coordinates

I have something like:

class User < ApplicationRecord

  has_one_attached :avatar do |attachable|
    attachable.variant :large,  resize_to_limit: [300, nil]
    attachable.variant :medium, resize_to_limit: [100, nil]
    attachable.variant :small,  resize_to_limit: [ 50, nil]
  end

end

How do I create...

• such "named variants"
• that are (first) cropped by user-supplied coordinates such as: [x, y, width, height]
• while keeping the uploaded (original) file unchanged?

Is it possible to "pass the coordinates to the model" somehow? If yes, how?

If not: Would the cropping need to happen separately in a controller action, creating a cropped version (of the original file), based upon which the "named variants" would be created? If so, how would that look?

This was added to Rails 7 (https://github.com/rails/rails/pull/39135):

class User < ActiveRecord::Base
  has_one_attached :avatar, variants: {
    thumb: { resize: "100x100" },
    medium: { resize: "300x300", monochrome: true }
  }
end

class Gallery < ActiveRecord::Base
  has_many_attached :photos, variants: {
    thumb: { resize: "100x100" },
    medium: { resize: "300x300", monochrome: true }
  }
end

<%= image_tag user.avatar.variant(:thumb) %>