It would at least tell him if the injection pump is functioning at all, I think.
Again I am not an expert but this us not mechanical fuel injection and AFAIK there is no "injection pump" in a Kubota common rail system.
In common rail a high pressure fuel pump (HPFP) maintains 5000 to 25000 PSI fuel pressure in the common rail. That pressure is maintained by a pressure regulator and rail pressure can be checked using a
fuel pressure test gauge. The ECU also monitors rail pressure in real time and sets diagnostic fault codes if its not correct. Those cades can be read using an appropriate scan tool.
The injectors are piezo-electric devices that inject that common supply of high pressure fuel into the combustion area of each cylinder. They are individually controlled by the ECU which uses signals from a multitude of sensors to determine the appropriate timing, pulse width, and magnitide of each injection event. On this engine there are four separate injection events for every ignition event and they are highly atomized and lightning quick (milliseconds). The injectors are tested and calibrated
on the bench using specialized test equipment that measures things like pulse width, volume. and response time. That data is then loaded into the ECU as a timing map for each individual injector.
Blindly playing with this system is simply not a good idea and messing with the high pressure side (red) is dangerous
Dan