If I understand you correctly, something else on the bus is the controller and you've got something presumably running LabVIEW embedded or something. I'm not sure if this will work, but a 30 second perusal of the NI-488 help indicated the function "ibrsv" which is described as "Request service and change the serial poll status byte.
ibrsv is used to request service from the
Controller and to provide the Controller with an application-dependent status
byte when the Controller serial polls the GPIB interface."
This may work for you. There's no LabVIEW primitive that calls this function so you would need to call the NI-488 DLL directly to access this function.
