I was able to solve it myself. Improvements/suggestions always welcome.

If you are going to use this code, don't forget to change the DocumentTypeAlias properties, the secret-crawler-useragent-id and TrySetTemplate(...)

using System;
using Umbraco.Core;
using Umbraco.Web.Routing;

namespace MyCustomer.Custom
{
    public class CrawlerAccess : ApplicationEventHandler
    {
        protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            PublishedContentRequest.Prepared += PublishedContentRequest_Prepared;
        }

        private void PublishedContentRequest_Prepared(object sender, EventArgs e)
        {
            var request = sender as PublishedContentRequest;

            if (request.PublishedContent != null
                && !request.IsInitialPublishedContent
                && request.InitialPublishedContent.DocumentTypeAlias == "article"
                && request.PublishedContent.DocumentTypeAlias == "loginPage"
                && request.RoutingContext.UmbracoContext.HttpContext.Request.UserAgent.Contains("secret-crawler-useragent-id"))
            {
                request.PublishedContent = request.InitialPublishedContent;
                request.TrySetTemplate("article");
            }
        }
    }
}

is working on a reply...