This is a very good idea and I'd love to see this script being created. This won't only help people a lot, it'd be a great feature even if you're not visually impaired at all.
Although, being a script developer myself, I know there isn't a built-in event in Plus! Live scripting which gets fired when a contact starts typing. It is possible to detect this through the Messenger protocol using a protocol sniffer, but unfortunately the Xniffer has some serious flaws causing it not to work properly on many systems. Therefore, we're all waiting for someone to create a proper protocol sniffer which can be used in Plus! Live scripting. When this would be released, a whole lot of new scripts could be made.
/ 
