Can I use STM to split test headlines in LPM?

You cannot do this because, for security reasons, we do not allow customer code to be run on our servers.  In the A/B Dynamic test customers can put anything they like (even a bot!) So if we allowed you to put the test snippet on a landing page - the bot could go live and wreak havoc!

So, the answer is to create one page, clone it, change the headline and put these 2 URLs in a Split URL test with the conversion code on the download page.