What is the best way to give SAAS subscribers their own A Record for logging in to their dashboard?
You can create a wildcard record in your DNS that resolves all *. DNS lookups to a single IP address, then identify the specific subscriber using the Host header sent from the browser.
EDIT: see http://en.wikipedia.org/wiki/Wildcard_DNS_record for more detail.
This is exactly the article you're looking for.
Note it is written for an older version of CI but the methodology should be intact.
If you plan to implement this, at least consider disabling it via options; either "profile exists" or "profile not exists, hidden or disabled" messages should appear at accessing the subdomain - else your client's security/privacy may be harmed.