If one has the DCS remote commands and a means to tx/rx with the TIU I can see doing this from software since the software "knows" there is a different engine control type and is controlling the interactions.
The DCS remote also "knows" there are two types of engine controls and it is communicating with each type, assuming the Legacy base is connected, etc.
I doubt the Legacy remote knows about anything that is not Lionel. Is it possible to hack the Legacy remote, or this idea to just try to broadcast DCS remote commands for each Legacy command that can be translated?