The CoffeeScript website says "If you'd like to create top-level variables for other scripts to use, attach them as properties on window, or on the exports object in CommonJS." Let me try to break this down, taking the CommonJS suggestion first.
Learning these solutions, and retro-fitting my little game to use a modern framework, is something that I knew I'd have to embrace. However, I wanted to explore how to attach variables "as properties on window." It turns out to be a "questionable" practice, the end-result being another global variable, but the actual code to add a window property was deceptively easy:
window.lb = lb