The following appears to work:
library("RMySQL")
con <- dbConnect(RMySQL::MySQL(), host = "aurora-db-in-aws.eu-west-1.rds.amazonaws.com",user = "username", password = "password", default.file = "/etc/mysql/conf.d/mysql.cnf", group="mysql")
dbDisconnect(con)
contents of mysql.conf
[mysql]
ssl_cipher = "DHE-RSA-AES128-GCM-SHA256:AES128-SHA:@SECLEVEL=1"