You cannot send reminder messages to specific Heartbeat channels, but you have the option to send reminder emails and notifications when you create a Heartbeat event.
Sending reminders by email
Reminders are sent as a push notification. To also send them by email, open Advanced Email Settings under Event Email Invitations, just above Event Reminders, and switch on Send event reminders as emails.
Reminder emails go to everyone invited except anyone who has RSVP’d "No". If Event Email Invitations is set to None, only members who have RSVP’d receive them.
If a reminder does not go out
A reminder only goes out if its send time is still in the future when you save the Event. Adding a 1 day before reminder to an Event that starts in six hours means that send time has already passed, so it is skipped and nothing is sent. Any reminders still ahead of the Event go out as normal, so on an Event six hours away, 1 day before is skipped and 10 minutes before still goes out.
When you are setting reminders close to the Event, pick a lead time shorter than the time left before it starts.