Another approach would be to write a custom router class extending EnsLib.MsgRouter.RoutingEngine, overriding OnRequest() method.

In OnRequest(), before calling ##super(), assign the JSON %DynamicAbstractObject build from the input request stream to a property.

Use the property in the router rules.

You can override the Page() method of %CSP.Page and redirect output to a stream, process the resulting stream and write it to the original device the page is using to send data back to client.

Here is a quick and dirty example, using IO-Redirect package available on OpenExchange.

The redirecting page : 

Class test.src.RedirectedPage Extends %CSP.Page
{

ClassMethod Page(skipHeader As %Boolean = 1) As %Status [ ServerOnly = 1 ]
{
    #dim sc as %Status
    #dim ex as %Exception.AbstractException
    #dim pageStream,processedPageStream As %Stream.Object
    #dim len as %Integer
    #dim buffer as %String
    
    s sc = $$$OK
    try {
    
    Set pageStream = ##class(%Stream.GlobalCharacter).%New()
    Do ##class(IORedirect.Redirect).ToStream(pageStream)
    $$$TOE(sc,##super(skipHeader))
    Do ##class(IORedirect.Redirect).RestoreIO()
    Set pageStream = ##class(IORedirect.Redirect).Get()
    $$$TOE(sc,..ProcessPageStream(pageStream,.processedPageStream))
    while 'processedPageStream.AtEnd {
        s len = 32768
        s buffer = processedPageStream.Read(.len,.sc)
        $$$TOE(sc,sc)
        write buffer
    }
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

ClassMethod ProcessPageStream(pageStream As %Stream.Object, Output processedPageStream As %Stream.Object) As %Status
{
    #dim sc as %Status
    #dim ex as %Exception.AbstractException
    s sc = $$$OK
    try {
        s processedPageStream = ##class(%Stream.TmpCharacter).%New()
        $$$TOE(sc,processedPageStream.CopyFrom(pageStream))
        d processedPageStream.Write("<div><span>original page had "_pageStream.Size_" bytes </span></div>")
    } catch (ex) {
      s sc = ex.AsStatus()
    }
    return sc
}

}

The original page :

Class test.src.OriginalPage Extends test.src.RedirectedPage
{

ClassMethod OnPage() As %Status [ ServerOnly = 1 ]
{
    &html<
        <div>
         <span>Hello, world, again</span>
        </div>
    >
    return $$$OK
}

To see the content, the CSP Gateway has an HTTP trace facility, see  documentation.

To access the response just before it is sent back to client, override methods of %CSP.Page