Changes required to Laravel to allow observing attached/detached events (https://github.com/laravel/framework/pull/14988#issuecomment-267717423)

AbstractModel.php

<?php

use App\Extensions\Relations\BelongsToMany;

abstract class AbstractModel extends Model
{

	/*
	 * This is copied AS IS from \Illuminate\Database\Eloquent\Model@belongsToMany but instead uses our own BelongsToMany class
	 */
	public function belongsToMany($related, $table = null, $foreignKey = null, $otherKey = null, $relation = null)
    {
		if (is_null($relation)) {
			$relation = $this->getBelongsToManyCaller();
		}
		$foreignKey = $foreignKey ?: $this->getForeignKey();
		$instance = new $related;
		$otherKey = $otherKey ?: $instance->getForeignKey();
		if (is_null($table)) {
			$table = $this->joiningTable($related);
		}
		$query = $instance->newQuery();
		return new BelongsToMany($query, $this, $table, $foreignKey, $otherKey, $relation);
    }

	/*
	 * Override Model@observe and add replace . with _ for the observer callback
	 */
	public static function observe($class, $priority = 0)
    {
        $instance = new static;
        $className = is_string($class) ? $class : get_class($class);
        foreach ($instance->getObservableEvents() as $event) {
			$methodName = str_replace('.', '_', $event);
            if (method_exists($class, $methodName)) {
                static::registerModelEvent($event, $className.'@'.$methodName, $priority);
            }
        }
    }
    
}

?>

BelongsToMany.php

<?php

namespace App\Extensions\Relations;

use Exception;
use Illuminate\Support\Collection;
use Illuminate\Database\Eloquent\Collection as EloquentCollection;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Relations\BelongsToMany as LaravelBelongsToMany;

class BelongsToMany extends LaravelBelongsToMany {

	public function attach($ids, array $attributes = [], $touch = true)
    {
		$returnVal = parent::attach($ids, $attributes, $touch);

		if ($ids instanceof EloquentCollection)
		{
            $ids = $ids->modelKeys();
        }
		else
		{
			$ids = [$ids];
		}

		foreach($ids as $id)
		{
			$attached = $id instanceof Model ? $this->find($id = $id->getKey()) : $this->find($id);
			if ($attached)
			{
				$this->fireParentEvent("attached.{$this->relationName}", $attached, false);
			}
		}
		return $returnVal;
	}

	public function detach($ids = [], $touch = true)
	{
		$results = parent::detach($ids, $touch);
		if ($ids instanceof EloquentCollection)
		{
            $ids = $ids->modelKeys();
        }
		else
		{
			$ids = [$ids];
		}

		foreach($ids as $id)
		{
			$id = $id instanceof Model ? $id->getKey() : $id;
			$this->fireParentEvent("detached.{$this->relationName}", $id, false);
		}
		return $results;
	}

	protected function fireParentEvent($event, $records, $halt = true)
	{
		$dispatcher = $this->getParent()->getEventDispatcher();

		if (! $dispatcher)
		{
			return true;
		}

		$event = "eloquent.{$event}: ".get_class($this->getParent());

		$method = $halt ? 'until' : 'fire';

		return $dispatcher->$method($event, [$this->getParent(), $records]);
	}

}

MyModel.php

<?php

class MyModel extends AbstractModel
{
  
    protected $observables = [
        'attached.someRelationship',
        'detached.someRelationship',
    ];
  
    public function boot()
    {
        MyModel::attached_some_relationship(function ($model) {
            // do something
        });
    }
  
    // the rest of your model
  
}

?>

so that is working)