Have you tried a Retrieve activity, by-association, and retrieve the headers fro $latestHttpResponse?
I googled and found this: https://github.com/mendix/RestServices/issues/4 , unfortunately it is for REST and contains a dead link on top, but maybe you can bend it to your will nonetheless :|
To solve this – The headers for the $latestHTTPResponse are stored in HttpHeader objects which have a reference to the response itself. You can use a retrieve-by-association to retrieve them:The headers for the $latestHTTPResponse are stored in HttpHeader objects which have a reference to the response itself. You can use a retrieve-by-association to retrieve them: