im afraid thats how it is when you have multiple clients. Some things can never be added for fear that others who use a different client cant use it. I dont see this feature having a large impact however.
Bama as for the sock portion its not that difficult if the person cannot be connected to then they simply dont get the typing msg. Another larger issue is this..
You in a channel of maybe 1500 people (unless you use several sockets only one at a time can send the typing method). If you do use multiple sockets how would you be able to cleanly display the some 84 people who are all typing a message? Im sure an echo wont do it.