You will have to replicate the feature code and change the reason:
exten => 781,n,Set(DB(fop2state/USER/${AMPUSER})=Out to Lunch)
exten => 781,n,UserEvent(FOP2ASTDB,Family: fop2state,Channel: USER/${AMPUSER},Value: Out to Lunch)
Chose an extension that will not overlap with your current dialplan.
Best regards,
