Why not monitor signals such as DTR, DCD, etc?
Case IO.Ports.SerialPinChange.CDChanged
Case IO.Ports.SerialPinChange.CtsChanged
Case IO.Ports.SerialPinChange.DsrChanged
Case IO.Ports.SerialPinChange.Ring
Since these pins apply voltage to ground for trigger, you could utilize a contact closure to either interrupt or apply the voltage, triggering a radiodj event?