I ultimately reached out to AWS support with this same question and they confirmed that it is not possible due to the fact that Google initiates the link between it and Cognito and therefore does not allow you to pass any data into that pre sign up trigger. I understand this is likely by design to not allow any additional data passed in with the google token to not allow anyone to manipulate the sign up process. Fully understandable from Google's perspective.

In terms of what to do, it's a pretty niche use case to require linking two accounts with different emails. Moving forward I will add the user's business email to the newly created social account as an attribute from the session once they are logged in. I will end up with duplicates of some users in Cognito but I will have to live with that. The social login account will be the one with access when the user logs in again. I could even run a lookup on users' attributes to find if someone has a separate account when logging in.