Some progress that may help others:
Looks like add_action('added_post_meta','hmm_update_meta_hook',10,4)
to hook the meta update. Not entirely clear on why but it works. That doesn’t solve my problem as the plugin code that does the meta updates doesn’t provide everything I need.
In the interim, I’ve just added a call to a routine external to the plugin and am passing $ipn_response. Works great. Entirely inelegant and unsupportable. ?? Really need a hook into the actual IPN response. Is that possible with this plugin?