What sort of scenarios would require the TRU to be locked out, and where is the line drawn between locking-out and accepting a delay and repairing/replacing the system?
The most common scenario would be no deployment of 1 reverser. The reasons for the fault maybe many and varied, but for example if it was traced to an actuator that say takes 3 hours to change it would be locked out and the MEL applied. The actuator would then be changed on an overnight hangar visit.
Where I work items are generally locked out/disabled to keep on time departures where possible. There are exceptions to this as well. For example an anti-ice valve maybe lock closed but the aircraft cannot fly within 5nm of rain. If this is not possible then the valve would have to be changed and the delay excepted.
Another example is a long haul flight with the IFE u/s. We will fix that and wear the delay. The pax would generally be happier with a 1 hour delay than no IFE for 12 hours. The line in the sand on this one would usually be made by an operations manager. There are crew hour limitations and schedules to take into account. So if the repairs might take 3 hours they might send the aircraft without IFE and offer the pax some kind of compensation. e.g a credit towards another flight.