I understand your motivation for the hive platform and I agree with that. But unfortunately, it doesn't have a great solution to provide push notifications to end user neither does splinterland chat.
I even have a discord server and notify people over there still time to time someone misses battles. When someone misses their battles in brawls their penalty in my guilds is they can't join the next brawls on day 1 and other players will have a preference to choose fray first on the last day if there are some frays still available. If someone still joins on day 1 then I or the officers remove that person from the joined fray.