Lemme get this right, you want an app to read your messages and then compose a reply on your behalf? There could be some fine replies coming from your phone.
I kind of get what he is saying, cause when I am on my laptop I would rather type then txt and a lot of my family have android and while they keep synced with google i.e google chat which it always does I can txt them from empathy on Ubuntu.
There is an app called blue phone elite which allows you to sync your iPhone (or any cell phone really) over blue tooth to a laptop. OS X supports voice over and could read you your incoming texts....not sure how well it would work or if Blue Phone Elite is still even being developed.