Render Output To String - Response.Flush Breaking Page Caching

I have some code that is used to replace certain page output with other text. The way I accomplish this is by setting the Response.Filter to a Stream, Flushing the Response, and then reading that Stream back into a string. From there I can manipulate the string and output the resulting code. You can see the basic code for this over at [URL] However, I noticed that Page Caching no longer works after the first Response.Flush call.

I put together a simple ASP.NET WebApp as an example. I have a Default.aspx with an @OutputCache set for 30 seconds. All this does is output DateTime.Now.ToLongTimeString(). I override Render. If I do a Response.Flush (even after the base.Render) the page does not get cached. This is regardless of any programmatic cacheability that I set. So it seems that Response.Flush completely undermines any page caching in use. Why is this?

extra credit: is there a way to accomplish what I want (render output to a string) that will not result in Page Cache getting bypassed?

ASPX Page:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="TestCacheVsFlush._Default" %>
<%@ OutputCache Duration="30" VaryByParam="none" %>
<%= DateTime.Now.ToLongTimeString() %>

Code-behind (Page is Cached):
protected override void Render(HtmlTextWriter writer)
{
base.Render(writer);
}
Code-behind (Page is not cached):
protected override void Render(HtmlTextWriter writer)
{
base.Render(writer);
Response.Flush();
}
Code-behind (Page still is not cached):
protected override void Render(HtmlTextWriter writer)
{
base.Render(writer);
Response.Cache.SetCacheability(HttpCacheability.Server);
Response.Cache.SetExpires(DateTime.Now.AddSeconds(30));
Response.Flush();
}