Well something like that is core to the password manager, and already introduced into the product since the beginning. If the maintainer has been competent enough to use (say) a secure RNG until now, he's not going to suddenly mess it up when adding a new feature.

Which is not to say it's a bad idea to get expert vetting for something like this (it's obviously an ultra-safe approach), but it helps to try to put things in context yourself, so that you don't have to find an expert every time you need to make a security decision. In the context of a desktop password manager, there isn't a terrible lot that can go wrong by accident and suddenly result in password exposure once the core product is formed and secure. If it happens, it'd be almost certainly due to a new maintainer coming along and somehow checking in unsafe code, rather than the current maintainers (say) suddenly forgetting they shouldn't call rand() or accidentally saving plaintext passwords on a disk.