Well, that is a strange requirement you have. I think you won't be able to limit the physical transport rate ovet the network, because of obvious reasons. The only thing I can think of is an abstraction lyer, that "surrounds" the network (that is the socket api) both on the server and the client side and simulates a data transmission at a given baud. Think of it as a pseudo-socket implementation, that uses the real sockets on the lower side and exposes a "constant speed socket" to the client.

Is this a way to go for you?