PTR record (rDNS) for multiple domains on a shared IP address
Solution 1:
Reverse DNS can only really have one name -- if you list more than one, DNS will just round-robin between them. So you just have to pick one "official" name for the server and always use that. All the other domains are, in essence, just aliases to the original.
Most services don't care what the name is that your server uses, they just care that the reverse lookup returns a domain name that resolve correctly going the other way as well.