This could be accomplished through a script using sed against a template w/your MONGO_OPLOG_URL and some AWS CLI commands. I don’t think it’s possible do it dynamically. Most of the issue is that the AWS instances run under local (secure) IPs and often you want to expose a public IP which you have to set explicitly.
I might be off-topic but just wonder if the performance improves with your cluster deployment and how can you set up MongoDB replica instead of MongoDB as a service here www.compose.io? Any resource is greatly appreciated.