Well I was drawing some concepts about this similar idea that I had, might as well say it now...
There are those USB adapter for the SIM card, which you put in, plug the adapter to your computer and with the help of included software you can send messages I think.
So if you can do that, you could most likely find/write a custom driver/application to interact with the dongle and have an automated SMS sending daemon.
But I am very unsure if that would work, because it's mainly designed for data transfer.
Another idea is that you could have a cheap dumb-phone connected via USB to your computer and with the help of software like MOBILEdit send messages. But I am unsure if it has an API to build on.