And I've got return values working! Well, all except floats and pointers, which should be pretty trivial, and structures, which I don't know how difficult it'll be. But I can query the width of a Raylib window!
In the above example, I had rl-call defined as (defn rl-call (name ret () args) (call !ffi-call (cons (!ffi-sym raylib name) (cons ret args)))).