HttpHandler EnableEventValidation Error

I wrote a HttpHandler to do some crazy thing or other. Put it in web.config's <httpHandlers> like this:

<add path="*/Pages/Review/*/*.aspx" verb="*" type="Freda.Classes.v3ReviewScreenHandler" />

Had to handle it in IIS7 too (or integrated pipeline mode on my workstation), so had to stick it in the <system.webServer>/<handlers> section like this:

<add name="v3ReviewScreenHandler1" path="*/Pages/Review/*/*.aspx" verb="*" type="Freda.Classes.v3ReviewScreenHandler" resourceType="Unspecified" preCondition="integratedMode" />

And to make both play nicely together, under <system.webServer> I had to also add:

<validation validateIntegratedModeConfiguration="false" />

The handler in this case is basically just a subclass of System.Web.UI.Page that dynamically loads a UserControl based on the requested URL path, or otherwise processes an ASPX page, which I did like so:

        public override void ProcessRequest(System.Web.HttpContext context)
        {
            string candidateControlFilePath = context.Request.Path.Replace(".aspx", ".ascx");

            if (System.IO.File.Exists(context.Request.MapPath(candidateControlFilePath)))
            {
                // Load Screen Control if available (the groovy new way)
                base.ProcessRequest(context);
            }
            else
            {
                // Revert to aspx mode (the non-bulk-printable way)
                Page page = (Page)System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(context.Request.Path, typeof(System.Web.UI.Page));
                page.AppRelativeVirtualPath = context.Request.AppRelativeCurrentExecutionFilePath;
                page.ProcessRequest(context);
            }
        }

And the "Load User Control" bit was like so:

        protected override void OnInit(EventArgs e)
        {
            base.OnInit(e);

            // Load ReviewScreen control
            string candidateControlFilePath = Request.Path.Replace(".aspx", ".ascx");
            v3ReviewScreen uctl = (v3ReviewScreen)LoadControl(candidateControlFilePath);
            this.Master.MainContentPlaceHolder.Controls.Add(uctl);
            uctl.Setup();
        }

And to make the Handler play nice with Sessions it was declared like so:

public class ReviewScreenHandler : System.Web.UI.Page, System.Web.SessionState.IRequiresSessionState

And all was dandy! Except for some funky GridView Row-Rendering stuff that was working in normal ASPX pages but not in my HttpHandler-based pages, producing this error:

RegisterForEventValidation can only be called during Render(); 

Which was weird, because the code to supress this sort of thing was already in web.config:

<pages enableEventValidation="false" validateRequest="false">

And I tried setting EnableEventValidation programatically in the HttpHandler's OnInit method, but got:

The 'EnableEventValidation' property can only be set in the page directive or in the  configuration section. 

And in the end the only solution was:

        protected void Page_PreInit(object sender, EventArgs e)
        {
            Page.EnableEventValidation = false;
        }

Which, called by magic and with no override or event handler to obviously hang off, is stupid. But works.