usb_moded does support that It uses /etc/modprobe.d/meego.conf for module loading parameters (standard linux module loading behaviour). So find in there the options line for g_file_storage and add ro=1
sed -i s/g_file_storage/'g_file_storage ro=1'/ /etc/modprobe.d/meego.conf
sed -i s/'g_file_storage ro=1'/g_file_storage/ /etc/modprobe.d/meego.conf