@vergilcw, every time the testing snapshot becomes the default one we send an announcement to all the users where we list all the version changes. They are encouraged (and given directions how) to check the release notes for any package they use in their work. In addition, they can use one of the functions I mentioned to freeze the snapshot in their script to be, say "2017-09-01". That way their code will run exactly the same at any point in the future. This is very similar to MS's checkpoint package but the users don't install any packages themselves.
That approach works most of the time but sometimes it becomes too restrictive. For some experimental work when the scope of a project goes beyond just R and requires carefully chosen versions of, say certain R packages, some system libraries, other applications and/or languages with their own packages (python?, java?) we use Nix package manager (https://nixos.org/nix) to create reproducible "sandbox" environments consisting of all the required versions of all the tools. Unfortunately such sandbox environments are not easy to access and manage from RStudio so the users use other means (emacs/ess, etc.).