If you don't need the PropertyUrl requirement, you can use a ContentReference instead and use an extension to get the simple url.

using EPiServer;
using EPiServer.Core;
using EPiServer.DataAnnotations;
using EPiServer.ServiceLocation;
using EPiServer.Web.Routing;

namespace ExampleExtensions
{
    public static class ExampleContentExtensions
    {
        static Injected<IContentLoader> _ContentLoader;

        public static string GetUrl(this ContentReference contentReference)
        {
            if (ContentReference.IsNullOrEmpty(contentReference))
                return "#";

            var content = _ContentLoader.Service.Get<IContent>(contentReference);
            var externalProperty = content?.Property["PageExternalURL"];

            if (!string.IsNullOrWhiteSpace(externalProperty?.ToString()))
            {
                return string.Format("/{0}/", externalProperty.ToString().Trim('/'));
            }

            return UrlResolver.Current.GetUrl(contentReference);
        }
    }

    public class PageBase : PageData
    {
        [CultureSpecific]
        [AllowedTypes(typeof(PageData))]
        public virtual ContentReference LinkToContent { get; set; }
    }
}

Thanks Brad,

I was looking for this kind of solution, but it was not working with PropertyUrl...

As I can see here they are using not only the Page references, but links to the catalogs (from EpiCommerce)... and I am not sure if they could reference here on other features of PropertyUrl... So I am looking the solution for PropertyUrl at the moment

I've not tested, but this may work for you. It should support Url, ContentReference, and IContent.

using EPiServer;
using EPiServer.Core;
using EPiServer.ServiceLocation;
using EPiServer.Web.Routing;

namespace ExampleExtensions
{
    public static class ExampleContentExtensions
    {
        public static string GetUrl(this Url url)
        {
            var content = UrlResolver.Current.Route(new UrlBuilder(url));

            return GetUrl(content);
        }

        public static string GetUrl(this ContentReference contentReference)
        {
            if (ContentReference.IsNullOrEmpty(contentReference)) return "#";

            var content = ServiceLocator.Current.GetInstance<IContentLoader>().Get<IContent>(contentReference);

            return GetUrl(content);
        }

        public static string GetUrl(this IContent content)
        {
            if (content == null) return "#";

            var externalProperty = content?.Property["PageExternalURL"];

            if (!string.IsNullOrWhiteSpace(externalProperty?.ToString()))
            {
                return string.Format("/{0}/", externalProperty.ToString().Trim('/'));
            }

            return UrlResolver.Current.GetUrl(content.ContentLink);
        }
    }
}

Thank you a lot, Brad!

I'll try this solution.

Upd: It works fine. Thanks!

Still could not mark Brad's post as an Answer...