Add a canonical URL for the ProductVariant entity

Hm... I don't really think it makes sense for variations to have their own url...
Let's take t-shirts for example, I've never browsed a single site that offers a dedicated page for the RED small variation of a particular t shirt.

If you take books, you often have the ability to get the digital version, or the hard copy, having a dedicated page for the digital version or the hard copy wouldn't really make sense either to me?

So could you elaborate on:

There are many scenarios when the product variant wants to be displayed on its own

?

a) Product = a course at a college, Product Variant = a specific instance of that course.
b) Whatever scenario this person is using it for: 💬 Product Variation meta tags not working Active

I'm not saying every scenario needs it, but it should be possible.

Is there a hook or event that allows the entity's definition to be modified? I wonder if this could be done as a separate contrib module?

@DamienMcKenna: yes, this can be done via a hook_entity_type_build(), see the following code from the core action module:

/**
 * Implements hook_entity_type_build().
 */
function action_entity_type_build(array &$entity_types) {
  /** @var \Drupal\Core\Entity\EntityTypeInterface[] $entity_types */
  $entity_types['action']
    ->setFormClass('add', ActionAddForm::class)
    ->setFormClass('edit', ActionEditForm::class)
    ->setFormClass('delete', 'Drupal\action\Form\ActionDeleteForm')
    ->setListBuilderClass('Drupal\action\ActionListBuilder')
    ->setLinkTemplate('delete-form', '/admin/config/system/actions/configure/{action}/delete')
    ->setLinkTemplate('edit-form', '/admin/config/system/actions/configure/{action}')
    ->setLinkTemplate('collection', '/admin/config/system/actions');
}

Closing this per #2.