Is a switch based Mixer possible?

Is it possible to have a switch position determine a mixer output?

For example, the ESC’s on my fixed-wing vehicle operate in forward-thrust mode by inputting a signal between 1500 and 2000us (typical for reversing ESC’s). And reverse-thrust is between 1500 and 1000us. I would like to have the ability to actively decelerate the vehicle once it touches down on a landing. We have successfully implemented this with ardupilot. The way it works in ardupilot is either autonomously or manually. Doing it manually is achieved by flipping a switch which activates “use_rev_thrust” which re-maps the max throttle stick position to output 1000us instead of 2000us. Min throttle position always outputs 1500us.

Is it possible to create a mixer file that alternates between forward/reverse thrust modes based on the state of an RC input switch?

You can map a RC channel to AUX control using parameter RC_MAP_AUX1. Then use control 3 5 as specified here to alter the mixer output.
Unfortunately you won’t be able to reverse mixer output when using a Simple Mixer. Simple Mixer only scales, offsets and limits the inputs. Output is the sum of inputs. But perhaps there is another way that could work for you.