There's Class::DBI::Relationship::IsA. I can't vouch for it though. There is lots more discussion on the Class::DBI mailing list. Here are a couple of threads that might help:
http://lists.digitalcraftsmen.net/pipermail/classdbi/2006-March/001029.html
http://lists.digitalcraftsmen.net/pipermail/classdbi/2006-August/001262.html

In reply to Re: Inheritance and class hierarchies in Class::DBI by perrin
in thread Inheritance and class hierarchies in Class::DBI by Anonymous Monk

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