An ASP.NET Page is also an HTTP Handler through IHttpHandler
interface implementation and therefore is a valid candidate to use with the HttpContext.Items
collection by definition in the MSDN document.
Because Page Controls are effectively in a handler there seems no apparent risk to reference page controls from the Items collection.
My main concern was lifetime of control objects in context of the Page versus longer lifetime of the request/context the collection relates to. However Controls will still be disposed properly through IDisposable when the page is complete and the garbage collector will still clean up Controls when they are finally released from the Items collection which shouldn't be long after the page handler expires.