Yeah, that sorta makes sense now that I think about it. In CGI::Prototype::Hidden, I put:

sub plugin {
  my $self = shift;
  my $name = shift;
  return $self->new('*' => $self->name_to_page($name));
}
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which will return a new page object that first searches the page's lineage, then the plugin's lineage. Then I can call a subpage like:

[% INCLUDE subpage.tt self=self.plugin("subpage") other=parms go=here 
+%]
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and in that subpage, self.THIS and self.THAT can refer to either original-lineage methods or the mixin methods. Cool. And yet, the mixin can also have defaults and overrides. This even works for sub-sub pages.