It would be really nice to to have something like a --use-iam-role flag for s3put. This could only be used on ec2 instances, but would allow for easily putting files into S3 (assuming the role had S3 access) by looking to http://169.254.169.254/latest/meta-data/iam/security-credentials/<role-name> for the access credentials. It would spare users from having an extra script to parse the output of that call, and seed the credentials to the command.
Have you looked into the highly recommended AWS Command Line Interface already (based on botocore)? It supports IAM roles (see AWS Credentials), and also provides higher level S3 commands specifically supporting your use case like cp and sync, see s3 for a summary (the raw s3api remains available as well):
I have switched all AWS scripting to the AWS CLI, which covers almost all services in a very consistent way, with enhanced usability and value adding functionality (e.g. the fantastic JSON Output with query result shaping ) - accordingly I consider those comparatively few scripts within boto to be more or less obsolete and wouldn't expect them to be improved much more in fact.