I've been toying with this for awhile, and it's a tricky piece of work.
Basically you'd need to script from a management host, and get it to query the SnapMirror status. If they are all idle, then it can start the next in the queue, if the status is transferring, then it needs to wait.
Not a simple task I'm afraid.
You could apply for an eval nearstore license to up the streams?