Nope, it doesn't complain about u3g. The usb_modeswitch command works without any errors.
But how do you stop pfSense from using u3g when it attempts to bring up a modem that requires switching. For example, I have a ZTE dongle sat here and when I plug it in, u3g tries to do it's thing but it can't because it sees it as a storage device. I'm guessing when usb_modeswitch is configured with the relevant data package, this will intervene?
I could really do with some help getting the data package on to pfSense to do a bit more testing but there isn't much documentation available and i'm a bit of a noob when it comes to FreeBSD!