AFAIK inheriting from the parent  site is the default  setting, so as long you don't set  anything explicitely, your site  should inherit  the AvailablePageLayouts from its parent.

if no one has explicitly set  any page  layouts in child sites, it should inherit  by default. And, by definition, if you're creating a site  from a template, you wouldn't have any child sites yet. However, if you ever wanted to write cod that would explicitly set the available page layouts  (other than in the onet.xml file like you were noting), you could use the PublishingWeb.SetAvailablePageLayouts method. That method actually has a resetAllSubsitesToInherit property.