Found a solution I think? You can send existing leads through a Lead Gen Form by creating a test campaign and adding the new form to an existing Direct Sponsored Content piece, then previewing the ad and filling out the form with the existing test email address. This pushes an activity through and allows you to select the form from the Smart List filter picklist.
Step-by-step testing process for the LinkedIn side is here: https://www.linkedin.com/help/lms/answer/94217
Sort of ridiculous, but this is how we got it to work.
Thanks, Robert.