Considering the Librem5 is essentially Just Another Linux System, this would be fairly straight forward to implement... The most basic implementation would be something like a script that you run on your computer, that connects over SSH to the phone, and uses ofono's dbus interface to send a SMS.
I haven't been as excited for a device as I am now since the N900.
I haven't been as excited for a device as I am now since the N900.