If the ends of your conduit is the same as the thickness of the wall, square and you pull the form up tight to it you shouldn't get any concrete in it, maybe a little cream.
You could stuff it with balled up newspaper to keep it from getting filled if you did have a slip.
I was going to suggest the plumber's test caps too. They are designed to be glued in place for a nice seal, then they are just popped out with a blow of a hammer. All that is left is a thin ring around the edge of the pipe that won't interfere at all.
The plumbers test caps sound like a good idea but i'm going to use duct tape [one size fits all] cause thats what i have handy. Whether it's foam, styro peanuts, or newspaper, i think it's a good idea to fill the sleeve just in case.
If you use the test caps, you wouldn't be able to run the snap tie through the pipe, which is important to make sure your pipe stays where you want it.
