Hi Dominic,
I am happy you like FSClient. It is built ontop of freeswitch and freeswitch does support text messaging so its mainly just the interface that is needed. I have not heard a lot of demand for messaging support and its not something I personally have great need for. Given the fact the core supports it however it may not be a large project to add it on. If you have some programming experience you may try yourself. Otherwise I do not have plans to add it in the near term future but will add it to the todo list.