I've never implemented one, but surely an ircd could do this if it wanted to? When someone joins a channel, send them a certain number of lines of history straight away, etc?
How it actually accomplishes that within the framework of the "protocol," I don't know.
The Quassel protocol supports this, unfortunately it looks like the quassel team doesn't care about taking it further than where it is.
Senders – there's absolutely no reason to fake senders, since it was always valid for messages to come from people not in the channel.
In a future release InspIRCd will adopt support for the server-time extension which will allow us to get rid of the timestamp duplication if a user's client supports it.