How to implement Material Mask using Green channel

I’m reading in the Documentation herehow to setup the Material Editor with a material mask, in my case, to support metal and non-metal textures in a single material and using a four-channel texture map. In the example, the section describing how to setup the chair with metal frame shows the green channel being used to house the mask (alpha), evidently a way to avoid artifacts associated with simply using an alpha channel due to some compression issue with DirectX that green overcomes. The example also uses color in the green channel to control the color of the metal, which in my case I’m not interested in, simply wish to limit metalicity to a masked area. Whether I simply use an alpha channel to mask the metal sections or the green channel, given those metal parts in the scene contain RGB information and have no need to modify the color of the metal(s), how would I set up the nodes to accomplish this, ideally using the green channel?

If the whole scene requires RGB, is that to say the mask for metalicity can’t serve the dual purpose of masking and providing an editable color, as in the Documentation, that it needs its own alpha channel, be it green or alpha. If that’s the case, would I need to open the texture in Photoshop and move the green channel to an alpha channel, then build the mask in the green?, a simple switch? How then Would I then simply multiply the green channel linked to alpha in a Lerp node?