Request user_meta or user_attributes using a guest author id and get a post_meta value of the same name from the guest author profile.
Doc block edit for filter approach
Alternate method for getting user meta/attributes. This requires a call to an alternate function instead of hooking into and overriding a function that may already be in use, but it does have the advantage of allowing you to pass in the full object instead of just the id, with is the only way to make 100% sure that you are getting the right data set.
Don't forget $single. And uhm… if the variable doesn't exist, you''ll never get a chance to see if is an object.