@cdswindell have you considered interfacing with the LCS system through wifi? Most raspberry pis have wifi these days and it eliminates the need for the SER2 and a second pi. Perhaps a second interface option on the back end would be a nice extension. From the LCS partner documentation, it seems like wifi is the preferred method anyway.
I'm very encouraged by your rapid progress here. A DIY hardware remote that is powered by a raspberry pi (or other wifi enabled python interpreter) now seems well within reach. It could be something as simple as a Cab1L with an aux3 and a whistle slider, or even more complicated like a Cab2 with the context sensitive touch screen. A raspberry pi could easily power a touch screen gui.