Hello!
I'm super busy right now so here's just a quick answer. Check smallUML:�
http://squeaksource.com/@ove2LM8opVGNawjT/fA1QDg4I, it's a project I did some time ago to build UML diagrams. It already deals with the mouse event handling for drag and drop for example.�
The diagrams are saved as text in methods, and you can rebuild a diagram from that, so maybe it's useful for what you want to do (you just send the text that will rebuild a diagram to the other user).
Feel free to ask questions here about this and I will try to answer!
Cheers,
Carla.
PS: Google "smallUML + Pharo" to read past emails about it