This
EDIT: I just did a flight and the transponder mode switch is not working. I also noted that the avionics power switches make Mobiflight disconnect from the sim; and I have to restart mobiflight. Thes 2 things are totally new since the last @MobiFlight update. Maybe Christian has an idea of what’s going on.
2nd EDIT: it seems that the original event for the avionics buss 1 and 2 isn’t there anymore. For some reason they both are FSUIPC-EventID, FCS_NEXT_TARGET. Whatever that is.
Yes that’s was my experience as well. I tried all the transponder modes under the area you show. Could not get any to work. Aobo problem or Mobiflight. Not sure. Also built the transponder from the MF video and everything worked except changing the mode with the button. Guess we have to wait on someone to fix this. 
.
I found the issue with the avionics bus buttons. It would seem that there is new MSFS2020 events for them under the 172 specific events.
I tried those but they didn’t work for me.
Try again. Mine just started working for me again this morning.
I will tomorrow. Must be the magic update that fixed it.
This is what I have for the C172 transponder but nothing works.
It connected to a Leo Bondar board through a 5 position switch
This is the GNS430 and KAP140 for the 172 VR panel.
I combined them into one module since they will only be in the 172. Still waiting on some trim, Arduino mount, and buttons that I’ll pick up at the 3D printers tomorrow. They are both all programmed and ready to be boxed up with just a single USB cable. Total cost with 3D printed parts is about $75. The dual encoders were $45 of that price but, I really like the propwash encoders. Small and easy to install. I won’t take the items from the PA-28 panel until I’m ready to make the module mounts and supports on the new dual plane panel.
I got the (>A:TRANSPONDER STATE:1,enum) to work. 0 through 4. 
Sooo. I have been wondering how to swap panels from a 172 to a PA-28 for my VR sim. The Cessna switch panel and the Arrow panel are so different. I came up with a solution that works perfect. The avionics bays are completely modular and can be swapped in and out with any 6.5 avionics. The ‘below the deck’ panel will also be modular and can be swapped from Arrow to 172 quickly. Picture of the design will follow. I only have it on paper for now. The throttle quadrants are also swapped with the lower panel. For GA flying these are the best two planes for me. A simple fixed pitch with fixed gear and a complex with variable pitch prop and retracts. All POC builds are done and I’m starting the final build tomorrow. Sure do love this building stuff. The helicopter and nascar build are on hold for a couple of weeks now.
I start the wood work today on this.
Might seem like a lot of work (it is) for a VR sim panel but…
I like building.
All the programming with Mobiflight is done and tested. I have all the hardware and Arduinos ready now. So I’ll be taking apart my POC rig and using parts from it to finish this.
The trim wheels, Cessna throttle/mixture and avionics brackets are from thingiverse shares.
The lower assemblies will slide/lock into place with just a single USB for each.
Most of the avionics are stand alone as well.
I left room for an Air Manager gauge panel if I decide to go with that later.
For now this will be just for a NO MOUSE/NO KBD VR flight sim.
Maybe have 2 panels you can just clip in, and connect a few wires, have 2 automatic setting profiles too for the different panels.
I would need to duplicate the 530 and some other avionics and switches though. The top panel is permanent and just the bottoms would swap out. All the avionics have a single USB cable to plug in.
It looks like the units will slide in from the back. is that Right? Each rectangle is its own unit? So you can mix and match for each aircraft.
From the front. The bracket is the size of the opening and the bezel is 1/4 larger than the opening
Yes each unit it separate. The 430/140 shown has its own mini mega controller. The rest will plug into a header to three other controllers.
OMG it think I created a monster.
4 Arduino controllers
Leo Bonnard board
High end PC with triple monitors
Oculus Quest 2 for VR flight
Over 200 connections to get it all running. FFB yoke, dual GNS radios, Mobiflight, XPForce, voice commands, FS VR kneeboard.
Modular panel for two different airframes.
Sure glad I retired and have the time to build and fly this. 
Starting the final assembly. Waiting on some IDC connectors and ribbon cable. Everything will be modular and swapped by unplugging and moving.The throttle quadrants will also be swappable
