I don't think it's all that difficult. Message the top 100 contributors of every subreddit that goes dark to ask if they want to be mods, and if they say yes put them on a two month trial.

I remember when Ronald Reagan fired all air traffic controllers in the US, that definitely had me wondering about the wisdom of the move and yet it went off without a hitch. Not a single plane crashed on account of that.

https://en.wikipedia.org/wiki/Professional_Air_Traffic_Contr...

Top contributors might not be interested in moderating though. It’s a different kind of activity - like being a tennis player vs being an umpire; Don’t think Roger Federer was ever interested in being an umpire.

P.S. Also moderating anything on the internet has been derisively called being an internet janitor.