W# Macro to get a config object constructed before JS built. The macro can then be used to access the config from the W# JavaScript annotated code. The advantage is that we have full flexibility in getConfig as it is before JS compilation.

macro.fs

type private ConfigMacro () =
        interface M.IMacro with
            member this.Translate(q, tr) =
                match q with
                | Q.CallOrCallModule (_, []) -> 
                    let rec convert (obj: obj) =
                        match obj.GetType() with
                        | ty when ty.IsGenericType && ty.GetGenericTypeDefinition() = typedefof<list<_>> -> 
                            C.NewArray <| (obj :?> seq<_> |> List.ofSeq |> List.map convert) 
                        | ty when ty.IsPrimitive || ty = typeof<string> -> !~ (C.String (string obj))
                        | ty when FSharpType.IsUnion ty  -> 
                            let unionInfo, _ = FSharpValue.GetUnionFields(obj, ty)
                            C.NewObject [ "$", !~ (C.String (string unionInfo.Tag)) ]
                        | ty when FSharpType.IsRecord ty -> 
                            C.NewObject 
                            <| (Array.map2 
                                <| fun (field: PropertyInfo) value -> field.Name, convert value
                                <| FSharpType.GetRecordFields ty
                                <| FSharpValue.GetRecordFields obj
                                |> List.ofArray)
                        | ty -> failwith <| sprintf "Cannot convert type {%s} to Websharper JS expression" ty.Name

                    convert (getConfig ())
                | _ -> failwith "Config must be Call or CallModule"

Not working fully, discriminated unions with arguments aren't supported. It should be enough to work with json file with simple record type.