Depending on the complexity of the other class it might be even simpler to encode the name of the other class as an overridable method. So in XML::Twig you have:

sub elt_class { "XML::Twig::Elt" };

# in XML::Twig, every time it needs to create an element
$self->elt_class->new;
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Then you just need to document elt_class and have your subclass override it as appropriate.