It's okay to be honest, not everyone has a plan laid out perfectly. Say you've interviewed at a few places (not sure if you should name-drop or not), and briefly explain the types of projects you are looking for. I wouldn't go into too much depth since it is still speculative at this point, but do give some details that show you are actively seeking an experience that will benefit your medical career. I would also include something in the end along the lines of 'I will send an update letter when I have a position secured' as long as the schools are not completely adverse to updates.
In the meantime, go volunteer or shadow, so that if/when you send this update, it is more than a boring 'I got a job'.