The answer is simple... use Telegram.. and if a witness is not in telegram he gets voted out.
Most of them are there.. it is thin, cross platform, mobile, P2P, and more secure than slack.
It has notifications built in.. and we already have Sparko who developed a monitoring app that plugs into it.. so we can all keep an eye on each others operations.
We have a telegram room for Witnesses only already too.
I am still voted out so I guess my suggestion is moot.
Nonetheless.. this to me is the most efficient course of action... if you make joining telegram a part of being voted in then you will be left with only witnesses that are in there.. which to me seems about right.
I hope this has been helpful in some way.... votes can be removed for everyone who is not there now.. just like with the sudden fork those that couldn't respond within the hour lost their votes and were no longer witnesses.. likewise if witnesses are active they will know this is coming and get in telegram NOW.
My 2 BTS.