Herve,
I'm afraid that the ViewerX is pretty much just a VNC viewer ActiveX control that has no support for the functionality in question. Neither there are any other VNC clients that have support for such features. And most probably you will have to make several changes to the VNC Server code in order to support the features you are looking for.
If you are interested we might help you with implementing such project, we have huge experience with the VNC code.