@mdhughes Chez, Racket and Guile more or less do R6RS, but there are a dozen R5RS and already more R7RS-small than there are R6RS.
Libraries aren't defined in R5RS, but the number of ways it has been solved are: (1) Fewer than the number of implementations, (2) Not difficult to implement.
I don't expect a centennial program to have many dependencies anyway, maybe no transitive dependencies. I don't expect the dependencies to be moving targets either, so the centennial program might just vendor them.
No unicode is bad, but even though the standard says you don't have to support it, most R5RS de facto support it, so I expect you can also write to R7RS-ok-not-THAT-small and get away with it.