I think a bridge that sends Universal Remote LC or Bluetooth signals to the TMCC or Legacy base would be the most practical since it doesn't involve surgery on the Legacy/TMCC locomotive and all the negatives involved.
Good point! For some reason my brain at 5am after an all nighter of programming did not consider the value of the locomotive involved. I was too busy thinking about whether it was possible. Not to mention it would make sense for someone who owns a Legacy locomotive to also have the legacy base to run it. Considering that I already have that LC Gateway project of mine up and running, adding the ability to output serial to the Legacy/TMCC Base really is no big deal. Now I just need to get access to one to try it.