Why would you want PTR-Records for SPD-Records? Just setup a SPF-Record for the subdomains: http://www.openspf.org/FAQ/The_demon_question
I guess you are right, if it's going to be sending email, it's going to be required to have it's own domain for DKIM and everything else. Wow, when did things get so complicated? I guess spammers are making us go through all sorts of things.