it's difficult to change protocols without breaking backwards compatibility unless the protocol is specifically created to allow future changes in a way which notifies the older clients that its safe to ignore the new thing they don't understand.
Do you know of any such protocol? The closest I could think of is that newer clients supposedly ignore the filename because they depend on the port number, so the md5 could masquerade as that. Scripts which handle DCC RESUME have no way to know what that port number is, so if the filename isn't there, they don't know which open send to that nick is being resumed.
Also, since mIRC is resume-sending prior to the end-of-file for the receiver's partial file, neither the sender or receiver is currently setup to handle hashing a partial file, so that would need to be added somewhere into the md5 handler.
