You can add the domain.com along with a mysql.domain.com database address without affecting the original domain. You can then access the "DreamHost version" by applying the Hosts File workaround so your PC connects to DH rather than the original site while you're developing it. When they're ready to switch over all they need do is make changes to the DNS and the DreamHost version will "go live" as DNS propagation takes effect.
Remember to remove your Hosts file edit after DNS propagation.
Maximum Cash Discount on any plan with MAXCASH