> However, I can also foresee a lot of technical and legal complications, not to mention potential maintenance burdens.
Shouldn't have a maintenance burden. That burden will be extinguished with the corporation.
If I were you, I'd put it on github with a corporate account, leave a readme that it's abandoned and then mark the repo read-only.
Let (interested) customers know and encourage them to fork it. Disable issues and pull requests before you publish.
Alternatively, put a source dump on your website, and let people know they can put it on Github, but you're not doing it. If nobody republishes it before the corporate site goes down, it is what it is.
What you say makes sense if there actually is a corporate shield.
Because “startup” is often used in a weak sense only to mean “new business,” there may not be corporate protections for the beneficial owners of this startup.
If it is a Silicon Valley style startup, then the founders probably ought to talk to their investors because that relationship matters and the investors probably know something about open sourcing code bases from shut downs.