Can I use Laravel's database layer standalone?

IMO it's absolutely valid to transition to an OOP approach step by step.

To your question:

Yes, you can use Eloquent standalone.

Here is the packagist site: https://packagist.org/packages/illuminate/database Add "illuminate/database": "5.0.*@dev" to your composer.json and run composer update. Now you'll need to bootstrap Eloquent. (https://github.com/illuminate/database)

The following is copied from the repo's readme:

Usage Instructions

First, create a new "Capsule" manager instance. Capsule aims to make configuring the library for usage outside of the Laravel framework as easy as possible.

use Illuminate\Database\Capsule\Manager as Capsule;

$capsule = new Capsule;

$capsule->addConnection([
    'driver'    => 'mysql',
    'host'      => 'localhost',
    'database'  => 'database',
    'username'  => 'root',
    'password'  => 'password',
    'charset'   => 'utf8',
    'collation' => 'utf8_unicode_ci',
    'prefix'    => '',
]);

// Set the event dispatcher used by Eloquent models... (optional)
use Illuminate\Events\Dispatcher;
use Illuminate\Container\Container;
$capsule->setEventDispatcher(new Dispatcher(new Container));

// Set the cache manager instance used by connections... (optional)
$capsule->setCacheManager(...);

// Make this Capsule instance available globally via static methods... (optional)
$capsule->setAsGlobal();

// Setup the Eloquent ORM... (optional; unless you've used setEventDispatcher())
$capsule->bootEloquent();

Once the Capsule instance has been registered. You may use it like so:

Using The Query Builder

$users = Capsule::table('users')->where('votes', '>', 100)->get();

Other core methods may be accessed directly from the Capsule in the same manner as from the DB facade:

$results = Capsule::select('select * from users where id = ?', array(1));

Using The Schema Builder

Capsule::schema()->create('users', function($table)
{
    $table->increments('id');
    $table->string('email')->unique();
    $table->timestamps();
});

Using The Eloquent ORM

class User extends Illuminate\Database\Eloquent\Model {}

$users = User::where('votes', '>', 1)->get();

For further documentation on using the various database facilities this library provides, consult the Laravel framework documentation.

Bastian's answer is pretty well. But there is one more thing. It can't work without Event Lib. To install it;

composer require illuminate/events

You are good to go