Hi Dan, I have a feeling what you meant to say above was that you can't leverage the champion/challenger test since it'll be a batch send. One workaround I've found for this is to use a batch campaign to add people to a list, and I have an additional triggered campaign when someone is added to the list which sends my email with a champion/challenger test. I also recently did a "if random sample is 50%" in the flow step to only add 50% of people to the champion/challenger send... The winning subject line was then sent in a separate campaign.
The only downside to this is that Marketo has been running slowly recently, and this makes things PAINFULLY slow. But, it's a workaround for the limitation that Marketo has presented us with.