Are there any examples of this special dual-license that you mention? (For example, going from AGPL to MIT triggered by the closure of a company.) This is an intriguing idea and I wonder if it has been tried.
I don't know of an example but you should talk to a license lawyer and create a new license. Basically it's similar to "normal" Dual-licenses that say the code is GPL but you can buy a MIT license from the maintainer. In this new case either the license would state that the rights are the same as GPL until the company goes out of business (and then it would switch to MIT)