It should be possible to reach the goal you describe, but it would most likely require creating a bridge between passbolt and gnupg, and setup gnupg to use the Yubikey as an OpenPGP Card.
This is possible in practice as demonstrated by Mailvelope project that accept gnupg as a backend (we actually participated to that work). However this is not a trivial task to implement / setup in a way that is user friendly for non expert users.
The team is open to it, but it is a significant challenge which we do not plan to tackle in short term as they are more important and lower hanging fruits.
