I have been trying to find a fix for this same issue for a couple of days now. I was able to resolve this by adding a key within the appsettings tag in the web.config file. This should override the .dll file when using the helper.
<configuration><appSettings><add key="loginUrl" value="~/RedirectValue.cshtml" /><add key="autoFormsAuthentication" value="false"/></appSettings></configuration>