I think there are 2 overlapping aspects to this conversation. 1) What it is, 2) How to do it. While there is a necessary and useful back-and-forth between the What and How, it seems threads like this get bogged down.
I believe the What needs to play out more. In other words, I for one would like to hear more ideas on What guys would want to do with control of rolling stock. I am 100% on-board with the original idea of sync'ing the directional or classification lighting of a slave/trailing engine to the master/lead engine. But I'm curious about other animation functions slaved to engine behavior...or that would make sense to be remotely-controlled via the engine.
Separately, there's How. As you've mentioned earlier, the challenge with the 99 cent "garage-door-opener" modules is how to shut up the transmitter so that the RF channel is available for other layout devices. This being in contrast to Bluetooth, Wi-Fi, or similar wireless modules/technologies that have the inherent ability to co-exist or play well with others. As you point out, what's neat about the so-called "learning" receiver module you found is the latching mode. This mode was not available on the original eBay 99 cent receivers. So with latching mode, you can send a short RF signal to turn on the back-up light and it latches on...and then minutes or hours later send a separate short RF signal to turn off the back-up light and it latches off. And for all the time between direction changes, the RF channel is available for other devices on the layout to do the same.
What's not clear to me is how a custom 50 cent microcontroller would play into a practical solution for the average O-gauger. Would this be a one-off proof-of-concept or would there be an actual order-able, no-soldering-required kit? Or something in between... like a link to a circuit board layout that one orders from OSH-Park and maybe a downloadable software image that runs on an Arduino Nano. I initially thought a rudimentary implementation would be some kind of discrete timer circuit (no software) that momentarily pulses the transmitter with the ON/Reverse command when it detects the engine's backup light turning on. The timer circuit would additionally momentarily pulse the transmitter with the OFF/Forward command when it detects the engine's backup light turning off. The receiver module would be set to the latching mode.
