What about a class to share resources, such as strings for example, both for client and server side. The problem is that there are cases when similar text is to be shown by client and server classes (in client - by drawing on some window, in server - by sending a broadcast message), and then you have to define similar string constants both in client and server, which is not very good IMO.
I do not know whether I can use such classes as HexenDefs for this purpose; if I understand this right, there's no HexenDefs object really created.