Is this what you had in mind? As far as I can tell it works and works in the same way.

```lisp
(defun projectile-project-buffer-files ()
  "Get a list of project buffer files."
  (let ((project-root (projectile-project-root)))
    (->> (projectile-buffers-with-file (projectile-project-buffers))
      (-map (lambda (buffer)
              (file-relative-name (buffer-file-name buffer) project-root))))))
```