Programmatically "unplug and replug" a USB device to load new driver in OS X?

IOUSBDeviceInterface187::USBDeviceReEnumerate() will do what you want. The only hitch is that to find all of the devices of interest and call this on them manually with IOServiceGetMatchingServices().

/*!
@function USBDeviceReEnumerate
@abstract   Tells the IOUSBFamily to reenumerate the device.
@discussion This function will send a terminate message to all clients of the IOUSBDevice (such as 
            IOUSBInterfaces and their drivers, as well as the current User Client), emulating an unplug 
            of the device. The IOUSBFamily will then enumerate the device as if it had just 
            been plugged in. This call should be used by clients wishing to take advantage 
            of the Device Firmware Update Class specification.  The device must be open to use this function. 
@availability This function is only available with IOUSBDeviceInterface187 and above.
@param      self Pointer to the IOUSBDeviceInterface.
@param      options A UInt32 reserved for future use. Ignored in current implementation. Set to zero.
@result     Returns kIOReturnSuccess if successful, kIOReturnNoDevice if there is no connection to an IOService,
            or kIOReturnNotOpen if the device is not open for exclusive access.
*/

IOReturn (*USBDeviceReEnumerate)(void *self, UInt32 options);

Look in IOKit/usb/IOUSBLib.h