Is there a postgresl server configuration that will let me expose an untrusted language like pl/R while still maintaining security?
I don't think that there is any general method to do this.  (Otherwise someone probably would have implemented a trusted version of the language using that method).

You could create a hot-standby database on a separate machine (or at least running as a separate and little-privileged OS-user) and let your users have at that with superuser permissions.  Note that they would be able to do nasty things to each other if they are clever and malicious enough, but should have limited capability to mess with the primary server.

Since a hot-standby is read only, you would have to set up the (super)user roles you want in the primary server, but disable them in the pg_hba.conf file.  Then use a different pg_hba.conf in the standby which allows those roles to connect.  

I think it is kind of playing with fire to do even this much, though.