How do I override a non-visible function in the package namespace?
Use fixInNamespace. :)
fixInNamespace("predict.ar", "stats")
or
fixInNamespace("predict.ar", pos="package:stats")
(Several years later...)
From Nicholas H's comment: if you want to push some code to CRAN that depends upon an internal function from another package, it will throw a build warning and be rejected by R-core. If you want that internal function, you should just take a copy of it using the ::: operator and maintain it yourself.
predict.ar <- stats:::predict.ar