I don't think the SMS driver will route SMS over the IP connection. In other words it just sees the Cradlepoint as a router and doesn't really know about the USB device being used for the broadband connection.
I have used my USB air card from AT&T connected to my laptop running a station to send SMS text messages using the SMS driver. However, I am fairly sure that if you are using an embedded JACE that it would require either using the Tridium GPRS modem option card or a third party GPRS modem connected to the JACE's serial port.
Not everything that can be counted counts, and not every thing that counts can be counted - Albert Einstein