This should work in Rails 3.2 and Rails 4:
post.association(:tags).add_to_target(Tag.first)
See this gist: https://gist.github.com/betesh/dd97a331f67736d8b83a
Note that saving the parent saves the child and that child.parent_id is NOT set until you save it.
EDIT 12/6/2015:
For a polymorphic record:
post.association(:tags).send(:build_through_record, Tag.first)
# Tested in Rails 4.2.5