Yes, that could work, and if no other solution presented itself, I would consider paying for a 3rd party service to do this. Current MailChimip basic service covers 2K users for free, and goes up to $55 / month for 5K to 10K users.
From a speed perspective, if I see a topic on my mobile device and I want to send it to the community, the steps to blast it out through a 3rd party service would make it a bit cumbersome. Having a full time-job while running a community on the side, I’m always going to look for efficiencies :). So if I’m not going to able to make good use of the $660 / year, not sure it’s a wise spend.
I thought this could be a way to help drive community engagement for sites driven by news, and help the sites grow. Realtime push of something relevant for the community, at a timely moment, could keep users active. Currently, we have digests which showcase what is popular, but by the time the email gets sent, the topics featured may already be a bit outdated.
Could be a paid feature for Discourse Enterprise. Something to think about.