Under the standard single instance I would mount the S3-Bucket with the following command using Goofy (or alternatively s3fs)
my-ec2$ ./go/bin/goofys mybucket my_mountdir
How can I do that under StarCluster? Does it allow S3 mounting at all? Or is it just for EBS?
What do you mean by "doing that under StarCluster"? Do you mean "automatically with StarCluster configuration, without logging in the instance and running the command manually"? If so, you may want to look at creating a plugin and run it. It's fairly easy.
The plugins bundled in StarCluster should serve as good examples.
https://github.com/datacratic/StarCluster/tree/vanilla_improvements/starcluster/plugins
I would create a plugin that, for each new node, runs the shell command you need to mount your s3 bucket.