Good idea but you know what, the bank can just deny that you've been hacked if someone sneaks to your passphrases.
I could put passphrase inside of tamper evident bag in the safety deposit box to show if it has been opened or not. But of course I am interested in the banks procedure about is there any chance for their employee to get access to whats inside of box without the customer being there.
You've got nice and plenty of idea for your security. If that's what's going to make you feel better in securing your passphrase through keeping it through the bank them it's up to you. But still think of it many times because there could be some tricks that they might do that you don't know to open it up.
I'll stick to what I think is better IMO and that's to keep it to myself and I know that where I can keep it and there's no one that shall sneak into it because I'll be the one building it.