Hi @Michael Davidovich - it's been a while! Here's a quick sample for how I'd do this:

Class Mike.Demo.REST Extends %CSP.REST
{

/// This method gets called prior to dispatch of the request. Put any common code here
/// that you want to be executed for EVERY request. If pContinue is set to 0, the
/// request will NOT be dispatched according to the UrlMap. In this case it's the
/// responsibility of the user to return a response.
ClassMethod OnPreDispatch(pUrl As %String, pMethod As %String, ByRef pContinue As %Boolean) As %Status
{
    #dim %request As %CSP.Request
    Set pContinue = 0
    Set version = %request.GetCgiEnv("HTTP_X_API_VERSION","unspecified; use X-API-VERSION header")
    Set class = $Case(+version,
        1:"Mike.Demo.v1",
        2:"Mike.Demo.v2",
        :"")
    If (class = "") {
        Set error = $$$ERROR($$$GeneralError,$$$FormatText("Invalid API version: %1",version))
        // Shoud be HTTP 400, but you probably want to report this differently/better.
        Do ..ReportHttpStatusCode(..#HTTP400BADREQUEST,error)
        Quit $$$OK
    }
    
    Quit $classmethod(class,"DispatchRequest",pUrl,pMethod,1)
}

}

Class Mike.Demo.v1 Extends %CSP.REST
{

Parameter VERSION = 1;

XData UrlMap [ XMLNamespace = "http://www.intersystems.com/urlmap" ]
{
<Routes>
<Route Url="/version" Method="GET" Call="GetVersion" />
</Routes>
}

ClassMethod GetVersion() As %Status
{
    Write {"version":(..#VERSION)}.%ToJSON()
    Quit $$$OK
}

}

Class Mike.Demo.v2 Extends Mike.Demo.v1
{

Parameter VERSION = 2;

XData UrlMap [ XMLNamespace = "http://www.intersystems.com/urlmap" ]
{
<Routes>
<Route Url="/version" Method="GET" Call="GetVersion" />
</Routes>
}

}

The standard approach for this in ObjectScript is the $$$FormatText macro - for example:

Class Demo.Text
{

ClassMethod Sample()
{
    Write $$$FormatText("Watch out %1, it's a %2!","Superman","large pizza made of Kryptonite")
}

}

Results in:

d ##class(Demo.Text).Sample()
Watch out Superman, it's a large pizza made of Kryptonite!

Set source = "NAMESPACE1"
Set target = "NAMESPACE2"
Kill ^|target|oddDEF(classname)
Merge ^|target|oddDEF(classname) = ^|source|oddDEF(classname)
New $Namespace
Set $Namespace = target
$$$ThrowOnError($System.OBJ.Compile(classname,"ck"))

EDIT: Don't do this. Everybody should ignore me and do what @Chad Severtson said instead.

You can use parameters on the return type. For example:

Class DC.Demo.SqlProcCollation
{

ClassMethod Test() As %String [ SqlProc ]
{
    return "Abe Lincoln"
}

ClassMethod Test2() As %String(COLLATION="SQLUPPER") [ SqlProc ]
{
    return "Abe Lincoln"
}

}

Given that:

select DC_Demo.SqlProcCollation_Test(),DC_Demo.SqlProcCollation_Test2()
where DC_Demo.SqlProcCollation_Test() = 'ABE LINCOLN'

Returns no results

select DC_Demo.SqlProcCollation_Test(),DC_Demo.SqlProcCollation_Test2()
where DC_Demo.SqlProcCollation_Test2() = 'ABE LINCOLN'

Returns 1 row