Geoff, as I understand it, it cannot happen as 3D cockpit gauges are a completely different animal from 2D panel gauges. 3D is designed as 3D with polygons whereas 2D gauges just use 2D art bitmaps. So for 3D gauges, while you can extract them from their native 3D environment there is no way to use them in a 2D fashion. You can instead however use some 2D gauges in the 3D cockpit providing the intended polygon in the virtual cockpit is sized large enough (and is flat) to place the gauge on it. But there are limits to this. And in this case I am also referring to Prepar3D, FSX, FS2004, etc here. Many people have done this in the past, for example using Garmin 430 / 530 units in place of old radio sets.
Secondly, because the new MSFS 2020 is a totally new platform made by Asobo with Microsoft's sticker on it, we can only expect that things will be a lot different. My hopes are that under Microsoft's direction they put things in place early on to be able to modify it so that it can eventually be used for panel building, home cockpits, multi-monitors, etc. I would assume Microsoft knows these aspects are a valued and desired aspect of the sim community and will get to it at some point. But we also have to think about all this from a performance aspect. Since it is a much more refined and graphic-intensive program it will undoubtedly take a lot to run it on more than one display.
With things being somewhat VR driven now as well most simmers being single monitor "desktop only" guys anyway, we know it is currently only designed for those aspects (and may stay that way). Regardless, even if multi-display expansion and 2D panels become available, the first thing to address will be how much more computing power, how many high-end video cards, etc, will it take to build a home cockpit, even a simple one. I think it will likely be one of those things that like with FSX, it took several years until the computers caught up to the sim software (although FSX was never fully optimized for multi-display simulation anyway). Luckily Prepar3D, being made of the same ESP core, does worlds better in that regard now but it also took them at least 7 years to get there. If Asobo does it my hunch is it will come along much sooner.
In the meantime, and provided enough video horsepower, people are just running a multi-display set-up in expanded desktop mode thus stretching the one window of FS across a few displays. I'm going to try this soon with a 32" 4k for my main display and a 29" ultrawide right above it. So the goal is to keep the cockpit panel on the lower display and the exterior visuals on the upper display. In theory, you could do this with 3 over 3 and somewhat maintain a 16:9 ratio but would likely need 3 video cards to do it. I also know some aircraft come with 2D panels for things like the G1000. I've seen a video where the MSFS2020 outside view is on an upper display and the two G1000 screens are on a lower display. I had asked about it and indeed it was an aircraft feature built-in and not some 3rd party software for the gauges to run. So that alone is good news for "native" interaction with 2D gauge panels.
For right now a lot of it really comes down to experimentation and see what works. Good luck.