Ok, so changing the service provider like so:

namespace App\Providers;

use Illuminate\Support\ServiceProvider;

use App\Support\UserMessage\UserMessage;
use App\Contracts\UserMessageContract;

class UserMessageProvider extends ServiceProvider
{
    protected $defer = true;

    public function register()
    {
        $this->app->singleton(UserMessageContract::class, function ($app)
        {
            return new UserMessage();
        });

        $this->app->alias(UserMessageContract::class, 'usermessage');
    }

    public function provides()
    {
        return [UserMessageContract::class, 'usermessage'];
    }
}

gives the desired result. The class gets resolved out of the IoC container and I can still use the alias, i.e app('usermessage');