I found dgx_donate_init_defaults which loads the default values to the empty sections of the email template. And a validate_page_slug_seamless_donations_admin_templates_callback which validates the submitted values and fires warning if they are empty.
There’s no hook for those functions, so I have to create a plugin to customize seamless donations plugin and hook the dgx_donate_thank_you_email_body to modify the email body before it is sent. It’s not a good solution but it’s possible and plugin updating won’t nuke me.