Cachè as a reverse proxy

I don't think it's totally stupid. We're looking at doing something similar with another technology (JReport).

Here's a code sample to help you get started - just change the port / page / filters / permission checks appropriately.

Class Demo.CSPProxy Extends %CSP.Page
{

Parameter HOST = "127.0.0.1";

Parameter PORT = 8888;

Parameter PAGE = "jinfonet/runReport.jsp";

/// Event handler for <b>PreHTTP</b> event: this is invoked before
/// the HTTP headers for a CSP page have been sent.  All changes to the
/// <class>%CSP.Response</class> class, such as adding cookies, HTTP headers,
/// setting the content type etc. must be made from within the OnPreHTTP() method.
/// Also changes to the state of the CSP application such as changing
/// %session.EndSession or %session.AppTimeout must be made within the OnPreHTTP() method.
/// It is prefered that changes to %session.Preserve are also made in the OnPreHTTP() method
/// as this is more efficient, although it is supported in any section of the page.
/// Return <b>0</b> to prevent <method>OnPage</method> from being called.
ClassMethod OnPreHTTP() As %Boolean [ ServerOnly = 1 ]
{
    Set request = ##class(%Net.HttpRequest).%New()
    Set request.Server = ..#HOST
    Set request.Port = ..#PORT
    
    // TODO: Add other stuff here, like authentication.
    
    Set page = $Piece(%request.CgiEnvs("REQUEST_URI"),"/"_$classname()_".cls/",2)
    If (page = "") {
        Set %base = $classname()_".cls/"_$Piece(..#PAGE,"/",1,*-1)_"/"
        
        // TODO: add query parameters from %request to the URL requested below.
        $$$ThrowOnError(request.Get(..#PAGE))
    } Else {
        Set fullPage = "http://"_..#HOST_":"_..#PORT_"/"_page
        Do ##class(%Net.URLParser).Parse(fullPage,.parts)
        
        // TODO: Better way of checking the requested resource.
        If $Piece($Piece(parts("path"),"/",*),".",2) = "jsp" {
            Set %response.Status = ##class(%CSP.REST).#HTTP403FORBIDDEN
            Quit 0
        }
        $$$ThrowOnError(request.Send(%request.Method,page))
    }
    Set %data = request.HttpResponse.Data
        
    // TODO: Do any other headers matter?
        
    Set %response.Status = request.HttpResponse.StatusCode
    Set %response.ContentType = request.HttpResponse.ContentType
    Quit 1
}

/// Event handler for <b>PAGE</b> event: this is invoked in order to  
/// generate the content of a csp page.
ClassMethod OnPage() As %Status [ ServerOnly = 1 ]
{
    If (%response.ContentType [ "html") && $Data(%base) {
        &html<<base href="#(..EscapeHTML(%base))#">>
        Do %data.OutputToDevice()
    } Else {
        Do %data.OutputToDevice()
    }
    Quit $$$OK
}

}

It is not a big problem to make it with Caché. But I would recommend you to look at different ways. Using Caché just as a reverse proxy, looks like a sledgehammer to crack a nut.

First of all, you should go from an original source of users, I don't see it in your question, but it can be LDAP/AD or something else outside of Caché. 

I would look at Nginx for example, which is actually very flexible.