just found that privy offers pregeneration of embedded wallets, I can compute it.. or they can.. after a proper request..

if customMetadata is including the custodial address, changing the custody addresses and calling this may be enough to get the address

there is a rest api I can call but ask for a secret and some app ids.. I them and I got a bearer but I would have to do trial/error to find all the parameters (username,fid,custody_address) and don't want to break anything.. so, will patiently await..

Alchemist. 
Block Builder. 
Creator.
Degen. 
Ethereum dev. 
Farcaster Framer.

https://docs.privy.io/wallets/wallets/create/pregenerate-wallets