I think I understand what the poster meant as I am looking for it myself.
What he want is, instead of typing/composing messages though his phone, he will do it directly on the computer and send it, of course it still uses the cellular capabilities of the phone but with less hassle.
This is already been done before by Nokia through their Nokia PC Suite (and do mind, it's not a smartphone era yet), I've tested it before and I'm sure it works and it's possible. Basically what happens is, you connect your phone to your computer through usb/bt on their PC suite and you can open their message center and directly compose messages from there and send. The message of course is still stored in the phone only you didn't compose in your phone.
You can google for send text through nokia pc suite and see what I mean.
Although at this point, I don't have any clues if this is possible in iPhone as I am still looking for myself. So if anyone has an idea on how to do it, please share.
Please correct me if I'm wrong and if it's ok to mention other brand name here.