Champion/Challenger Batch Solution #lessonslearned
There are a few articles in the community about the fact that you can't use Champion/Challenger testing on batch email campaigns. We came across this when using programs in our engagement engine - misread the article about using Champion/Challenger in engagement engine (ie can on a simple email, but not a program). We use programs in Engagement Programs for multi-touch attribution analysis.
We simply made the batch that was being used in the stream to have "Request Campaign" in the flow, with a trigger campaign in the program being used listening out for the request. We then set up that trigger campaign to send the email required for the stream - since when using programs Marketo is looking for program membership we then just have some camapigns to update status based on sent/del/open/clicked, so they won't get the same email on the next cast.
We're now getting A/B testing on any of our nurture programs we want.
It's worked for us, but before you try it out I recommend you test and use a sandbox if you have one.