After a bit of searching I didn't see this posted anywhere, and the official Sugar docs specify that the only allowed 'transport' values are 'http' or 'https'.
$sugar_config['full_text_engine']['Elastic'] = [
'host' => '<host>',
'port' => '9200',
'transport' => 'AwsAuthV4',
'aws_region' => '<region>', // Ex. us-east-1
'aws_access_key_id' => '<key id>',
'aws_secret_access_key' => '<access key>'
Verified to connect on Sugar Enterprise 8.0.x which ships with Elastica 6.0.1.