Insulating rim joist area with gap

I'm working on a room on the ground level which is partially below grade. I would like to insulate my rim joist pockets with 2" foam board sealed with expanding foam.

My dilemma: The wooden sill plate doesn't run from the top of the inside wall all the way to the back of the joist pocket - there is a 1.5" gap between the inside of the inside wall, and the inside of the exterior brick. So the joist pocket "floor" or sill plate, isn't continuous from front to back. Looking down, I can see the inside of the exterior brick wall down to the concrete foundation.

My idea: I don't want to fill this gap, I just want to cover it - possibly with 6 mil poly to prevent any moisture from reaching the back of the joist pocket behind the foam board.

Building is a side split - late 70s. Can any one comment whether they've seen this before, and whether my solution is appropriate / needed?

Thank you.