When you say this
The caveat is that these users are ...
But these are not users, then just email addresses, right?
Assuming that is what you meant, then the three choices I see are
1. Make a report of Reminders due to be sent, and make a formula URL button to update a date field to trigger the OPEN Notification and refresh the report with that record gone. Then the user would receive a morning Subscription email and just mindless click a button on all the records until they are all sent. So many clicks but pretty mindless and quick.
2. Contract with a QSP like myself to have the same thing done, but by clicking a single button once which would run a script to fire the individual emails by updating a date field on each respective record. One click.
3. Contract with a QSP like Juiced to install their "Triggers Add On" to run a process automatically each day. No Clicks.
