A couple of words comes to mind - Very Dangerous Procedure.
Welding on a closed vessel pressurized with any gas is very dangerous. Should the welder burn through the run pipe wall, you would have an uncontrolled release of the stored energy of the nitrogen gas.
From API 1104 19th edition
B.3.1 WELDING OF TEST ASSEMBLY
For in-service welding, pipeline operating conditions that
affect the ability of the flowing contents to remove heat from the
pipe wall should be simulated while test joints are being made.
Note: Filling the test section with water and allowing water to flow
through the test section while the test joint is being made has been
shown to produce thermal conditions equivalent to or more severe
than any typical in-service welding application (see Figure B-2).
Welders qualified under these conditions are therefore qualified for
any typical in-service application. Other media (e.g., motor oil) may
be used to simulate less severe thermal conditions.
For heat input control procedures, the welder should be
able to demonstrate the ability to maintain a heat input level
within the range specified. For temper bead procedures, the
welder should be able to demonstrate proper bead placement.
Does the client have any means to control or vent the gas in the event of over pressurization?
I would probably place a good deal of mileage between myself, my company and that particular customer. I personally would not want to expose myself to the potential liability or injury in this "experiment".
Just my opinion.
Al