In this case, the only option would be a middleman frontend running 24/7, fetching all automatically generated events.

Of course, I see your point, since being online with your smartphone day in day out or having your workstation at home online all the time can not be considered a good solution for this type of problem.

I guess, in the case of pure user-triggered actions, the middleman approach would be adequate, since it could be done with a very tiny and straightforward app, which could handle multiple third-party services.

However, for sever-side solutions like service-workers, we would definitely need an authentification interface on Steemit, or have a lot of trust in the third party :)