Just coordinate the release of the urls and the transfer of the instance.
I’m skeptical about this since you are squatting on at least 18 urls while trying to get volunteers to create value out of them. Nothing leads me to assume you are being altruistic.
Edit: misattributed something, woops
It seems like you are waiting for the next influx to potentially monetize and trying to hold the most potential instances without putting any work or money into it. It’s just my impression.
I also think instances without users are a terrible idea and I’d rather real instances come about organically instead created by people that actually care about the subject.
So are you willing to give up ownership of the url and have the instance be transfered to someone else’s hardware?
Maybe I misunderstood where you are going with this.