Query cname against RBLs

Dear spamassassin users,

I have observed that sometimes spammers register a bunch of throwaways 
domains (example.xyz), which are all CNAME towards one main domain 
(example.org). I'd like, with spamassassin, to query those CNAME (for 
instance that example.org) of URIs found in the mails against some RBLs 
(for instance URIBL, but others too).

I've tried and tried and couldn't make it work. Do any of you know of a 
way to do this, or would one need to write a plugin for that?

Thank you!
Laurent S.