First step, in all cases, is to add the domain and subdomain to DreamHost and add hosting to the subdomain. From there, there's a few possible paths forward:
[*] Probably the cleanest solution would be to subdelegate the domain to our nameservers (by creating NS records for the subdomain on Route 53, pointing to our three nameservers). This would route lookups for the subdomain straight to our nameservers, and would additionally allow you to create sub-subdomains which would also be routed to our nameservers.
[*] Another option would be to create a mirror of the subdomain on a dreamhosters.com subdomain, and create a CNAME record pointing from the subdomain to the dreamhosters.com mirror. The mirror would share the same IP address as the "real" subdomain, so it'll work just as though you had the address set up directly. (The sole purpose of the dreamhosters.com mirror is to get a copy of the IP address up on a publicly accessible domain.)
[*] If you really want to avoid using our nameservers, you could create an A record with the IP address of the subdomain (which you can look up by clicking the "DNS" link for the subdomain). However, this address may change if we shuffle things around internally, so, unlike the other two options, this one would require some occasional maintenance.