Hi Jeff
Thanks for posting in the Community!
At this time it is not possible to use Terraform instead of CloudFormation to create a Cross-Account Role. I'm sorry!
If the ability to do that is important to you I would highly recommend sending feedback to our Product Development Team via our feature request form.
Any request feedback or comment submitted to this form is passed on to them directly for their consideration.
As a growing company we take our feature requests very seriously as we are continuously looking for how we can make our product more useful and effective for our users.
If you're interested to know what happens to your feedback once you've submitted it please review What happens to my feedback?
Thanks for your request!