I'd love Lionel to come up with a way to run older legacy engines on the Universal remote (UR). It would a cool way bring the whole legacy catalog together. I'm sure it's just a pipedream but it would be cool. Any one else think so?
I could rigs such a device up like that for sure. I have already been working on something I am calling the LC Gateway and it should certainly be able to do what your asking. First I am working to add features to the existing LionChief Remotes such as track switching, demonstrated here: https://youtu.be/ZtITYrEDdwc
But I have also figured out the code to make a train side module. I do not have any legacy equipment to test this with so for now it will have to wait until I join a club where I can play with legacy gear.
Aside from a module in the train itself. It would be much easier for me simply have my gateway board bridge the signals from the Universal Remote into the serial port of a Cab-1L or Legacy base. Would something like that serve the purpose you had in mind? or are you only interested in a train side LC upgrade module.