The freight depot is really an inspiration! Everything about it comes together to make a really convincing and satisfying result. Thanks, Keith, for documenting the process.
I agree that what Mike suggests is very feasible if perhaps somewhat time consuming to execute the application. I'll throw an idea out for consideration using 3D printing which seems so useful these days. Model the wall in 3D including the battens. Include the outline of the lettering as an outline molded into the 3D board and batten by projecting the lettering onto the wall. Then the letters on the printed wall can be filled with ink or paint that will be confined to the outlines. The other option would be to just project the entire letter shape into the board and batten surface and have very slight letter depressions that could be filled with paint. Just throwing it out there for another idea.
The lettering could be laser cut into RC airplane Trim Film. It is a very thin sheet of self stick material in that is used to cover RC wood airplanes. MonoCote is the brand I have used in the past to make graphics for my sons Motocross helmet. I have actually used different colors one on top of the other to do one color of letting on a different color of background. The laser can usually be set to only cut through one color and not the background. After cutting you then just peel away the parts covering the background that you don't want and you have it.