Yes, at this time we have not solved this issue.  Other initiatives have popped up.  That being said, would be good to know.  
 
In a parallel  topic, for whole page redirects, we have split testing using Cloudflare instead of Target (because of the delay and also the issue with Control getting more traffic).  I wonder if in a similar fashion you can pre-split for Drift testing and then based on an incoming URL query, you deliver different playbooks?