Report workflow on multiple records only passes one id...

I have a list of assessment records, and a report workflow that opens a stateless form so the user can choose who to assign one to and when it is due by. It also updates the status, and all works nicely, as long as it's one row at a time.

But when I try to do the same thing with an action button for multiple records, it only ever passes one record id through to the stateless form at a time. How can I guide users through updating several records at the same time please, without letting them loose on the whole bulk editing thing? I've tried setting the action item to execute For Each Record and For Collection of Records but neither option passes more than one record through.