So far, the only way I can think of doing this is by adding a rule, to the VPS firewall/iptables, that will only allow connections by all AWS East IPs on the MySQL port, and refuse all other connections.
Given the large number of IPs, is there a better way to do this.
NOTE: The VPS is not on AWS, so using their security rules is not an option.
Also, access will only be granted for a user created specifically for IronIO access.
UPDATE: As suggested by tadman, I've come to the conclusion that establishing a remote SSH connection is the safest route. I tracked down the Net::SSH and Net:SSH::Gateway gems that would make this possible, however I am still unsure how to do this using key authentication, so created a new question here.