Email Content Configuration

We would like to modify the content of some of the emails that are sent to the signers. As an example based on data in our application we would like to modify the content of the invitation based upon the signer being the primary person on the contract or a proxy for the person such as a legal guardian. 1) Can we modify the invitation email content, and others, via the REST API? 2) Can we modify the invitation email content per package when we create the package or prior to the invitations being sent? Thanks, Dave