MAybe a quick email to the manufacturer might be the fastest answer.

I would suggest looking at the applicable job specifications, pipe, fittings, gaskets, materials and the execution of such. If it doesn't 'specify' neverseize, it isn't suppose to be there. Additionally, it could be doing damage to the gaskets or material.
Another method would be to look at the submittals. If it wasn't submitted by the contractor and approved by the customer, it isn't allowed. If would need to be submitted in the correct section too. Just because the millwrights or iron workers have submitted it and can use it, doesn't mean the pipe crew can use it.

You could also submit an RFI to the EOR. If the engineer has no problem with it and states it in writing then you are covered.

Not being a smart Alec so please dont take this wrong. The code doesn't specifically prohibit using peanut butter on gaskets either.

You could ask the pipe supervisor to provide some sort of proof of authorization that they CAN use it. Until that is done, they should stop.
"Well, that's the way we've always done it" isn't sufficient.

Spiral wound gaskets are designed to deform as the flanges are tightened. In between the spiral windings is the gasket sealing material. The material used to be a now banned material, asbestos. Today an alternate proprietary material is used depending on the manufacturer and service requirements. As the gaskets are crushed during the tightening of the flanges, this material will crush out into the spiral grooves of the flange mating surfaces. Check the thickness of the spiral wound material prior to tightening the flanges then check the gasket thickness of a gasket that has been removed. Material such as never-seize could prevent the sealing material from filling the grooves in the flanges. These gaskets are also sometimes used on flat face flanges with no RMS finish though I would use a gasket material designed for flat face flanges. Also be sure to clean the surfaces of new flanges and flanges that have had gaskets removed prior to placing a new gasket. This should assure there is no debris, paint, or sealing material left on the surfaces. The spiral wound gaskets are intended for one time use only as they are intended to crush and the sealing material to extrude as the flanges are tightened.

I don't know if any code prohibits the use of never-seize material on the gaskets or mating flanges but I wouldn't use any sealing material on a properly tightened flange, put the peanut butter on your sandwich, it will keep the bread from sliding around.

As an additional note gaskets can be ordered from the manufacturer with different crush thicknesses. There are many specialty gaskets for different functions. Check with the manufacturer for a list of specialty gaskets.

Hope this helps.

First of all, the correct brand name is not "flexataulic" but "Flexitallic".
Second, enter their site (www.flexitallic.com/resources/product-brochures-catalogs) and download for free the handbook titled "Design criteria", with 71 pages.
It would be a surprise for me if all of your doubts aren't cleared up on the handbook.
Giovanni S. Crisi
Sao Paulo - Brazil

Pipe dope??? Never Seize for sure, but pipe dope? Wouldn't that do the opposite if what you want? (e.g. The gasket to release without a bunch of time with a gasket scraper when you break the flange apart.)