PB Classic uses ASP.Net web services. Support for MTOM and WS-Security were added to ASP.Net web services through the Web Services Extensions, a plugin for the Visual Studio IDE. Given that it was a plugin, it's not something that can be used by PB Classic.
The WCF web services in the PowerBuilder.NET IDE do support such standards. If you are looking to create a client, you could create a WCF client in PowerBuilder.Net and then expose it through a COM callable wrapper that PowerBuilder Classic could use.