The AWS libraries used by S3_PING do not support authenticating into the AWS API via the EC2 instance profile (implicit credentials stored on the AWS EC2 instance).
The AWS SDK for Java supports instance profile. This eliminates the need for storing AWS access keys in config files.

Are there any plans to add official support for this in JGroups?

Resources:
http://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/java-dg-roles.html
http://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/credentials.html

Example third-party protocols:
https://github.com/tagbangers/jgroups-s3-client-ping
https://github.com/mcacker/jgroups-native-s3-ping