Authenticating via Kerberos against a web service using a C++ client...
I just thought this information might assist in expediting your research into the matter.
When using the automatically generated proxy class template that is created when referencing a web service from a C++ client application, you may define the proxy as per the following example:-
The template uses the CSoapSocketClient class by default. This class only supports NTLM authentication with the web service.
If you require that the C++ client application to authenticate via Kerberos with the web service. Use the CSoapWininetClient as per the following example when declaring the proxy.
HRESULT res = svc.HelloWorld(&str.m_str);
The SecureSOAP Sample may assist you further with implementing a C++ client against a web service.