Hi Philip

The way the Redirect Url Management functionality works is the redirect is stored in a separate database table to the Umbraco Cache, and the Cache takes precedent, so if a route isn't in the Umbraco Cache, then just before serving a 404, Umbraco will look to see if a redirect exists in the umbracoRedirectUrl database table. So I think in your scenario you can just do the same eg:

   [HttpGet]
        [Route("get/?url={url}")]
        public object Get(string url)
        {
            var content = UmbracoContext.ContentCache.GetByRoute(url);
            if(content == null) {
                 //no matching published Umbraco content, let's see if a redirect exists:
                // Umbraco has a core RedirectUrlService that queries the db with a snappily name method called GetModeRecentRedirectUrl...

                 IRedirectUrl redirectUrl = Services.RedirectUrlService.GetMostRecentRedirectUrl(url);
                 // (now I'm not sure what format your url is in, but this method is expecting an Umbraco route, which if you have domains set will need the prefix of the route domain id see: https://github.com/umbraco/Umbraco-CMS/blob/v7/dev/src/Umbraco.Web/Routing/ContentFinderByRedirectUrl.cs#L23 for an example)

                  if (redirectUrl!=null){
                         // we have a redirect!
                         var targetRedirectId =  redirectUrl.ContentId;
                         content = UmbracoContext.ContentCache.GetById(targetRedirectId);
                         if (content == null){
                            //the redirect content doesn't exist!
                            throw new HttpResponseException(HttpStatusCode.NotFound);
                          }
                 }
                 else {
                       // no redirect, no content, return not found
                       throw new HttpResponseException(HttpStatusCode.NotFound);
                }
         }
            //somehow we have content!
            return new ContentDataModel(content);
        }

Anyway that's the gist if I've understood correctly!

regards

Marc