Cross account IAM auth - AppSync resource policy

AppSync is an amazing service for running GraphQL APIs. Just as with REST APIs when using API Gateway we should be able to call the GraphQL API from other systems which reside in other AWS accounts. I do not see a way to setup the cross-account bi-directional trust like you can do so easily with API Gateway via a "Resource Policy".
Does AppSync already support cross-account IAM auth somehow that I can't find in the AppSync Console or documentation anywhere? If not, I think AppSync is missing a HUGE use case of allowing "systems" (lambda, ECS, EC2, anything that uses an IAM role) to also be a AppSync client.
I can make this work if the calling client (lambda role) is from the same AWS account as where the AppSync API is running, but there isn't anyway to support this from a different AWS account that I see. I could see a B2B case where a business may want to expose their AppSync API to a partner who also runs in AWS or if a company just has multiple AWS accounts which is very common.
I'd rather not "assume" an IAM role from the AWS account where AppSync is running... that is just a PITA.
@awsed any thoughts on this? I'd be happy to discuss.

Thanks,
Erik

Start by checking the AppSync Console and documentation for IAM auth and resource-policy support, then compare the cross-account behavior described for API Gateway. Done means establishing whether cross-account AppSync clients can be supported and defining the required resource-policy or trust configuration.

Written by the indexing model from the issue text.