I wonder how much Bitcoin one of the big custodians would have to lose in order for them to make an attempt at a fork to get it back.

Supposedly BlackRock has 740k bitcoin. They are using at least two custodians , but presumably there is some amount where if the custodian lost it, BlackRock would consider it better to try to push a hard fork giving them a do-over than to absorb the loss (reputationally or financially).