You are killing us Mike! :) We really don't like to think that BWS is
a webwallet. Note
that private keys are not stored (not even encrypted) at the server.

Sure, sorry, by web wallet I meant a blockchain.info/CoPay type setup where the client has the private keys and signs txns, but otherwise relies on the server for learning about the wallet contents. I tend to call wallets where the server has the private key BitBanks but I don't know if anyone else uses this terminology. It might just be a personal quirk of my own ;)

we think having some visibility of the wallet by the multisig
facilitator will make the user experience much better (e.g: mobile
notifications).

Fair enough. Yes, push notifications to mobiles in a decentralised way is rather a hard problem.

I think what Gregory suggested is then the best approach for you to do what you want. Whether it's worth the additional complexity is something I don't have any feedback on, only you can judge that.