If you can't replicate the fault, I guess it's not going to be easy...
You can can do other tests, like injectors return flow? You may find something, but you cannot guarantee that it is related to the problem you are trying to solve.
Most likely you don't have ambient data/freeze frame data for the code right? You could use a communication tool with PID logging capabilities, deliver the car to the customer, and ask him to start, and stop the capture!? You need to trust him a little to do that, but it may help to have more information of the conditions that originate the fault.
Have you tried monitoring the rail pressure whilst driving (prefably uphill) say in 2nd gear and wind the rail pressure right up to its maximum and hold it there for as long as possible. It should sustain max pressure. No decay at all. Basically you are loading the engine up and the injectors are held wide open for long periods. I have found numerous blocked fuel filters buy running that test. It's quick and easy and can be done using nothing but a scan tool. I know you have already replaced fuel filter but it may replicate the fault as you are stressing the pre supply system due to it having to be able to "keep up" with the demand from the high pressure side. Simply doing the high pressure tests in the workshop almost never shows this type of fault as you are not really checking the volume only the pressure capabilities. If you get the injectors wide open for long periods it may prove volume is good
Rolling road tuner with a big fan in front of radiator to keep it cool while running fast normally saves a lot of problems driving round the roads to find those types of faults, do you not have a local garage with a power rolling road to use?
Rolling road tuner with a big fan in front of radiator to keep it cool
If the fan stops working I can call you ??? ROFL !!
Perhaps you have not thought this through ?
If, that's a big if. A proper rolling road tuning firm I sure would not put their business in such a position to work with faulty equipment, would they!
The Alleged fault cannot be replicated on the road. I have loaded up the HP system to 2200 bar - well above what it is likely to be in normal service on the road.
It is time to walk away until such time that it gets worse and can be repeated at will. In its present state, if one was lucky enough to catch the fault and effect a repair then how long would one spend testing that repair ???
For all I know it could be a mystery shopper blowing wind up my shirt.
We can draw line under this for now. No Fault - No Fix