Maybe ten years ago, at the drivers meeting, they told us of a particularly rough spot on 1, down about the 3, that was about 30 feet wide and ten feet deep. They'd marked it with lots of green dye.
We drove the course in our chase vehicles right after the meeting, and sure enuf, there it was.
Made a really early run right thereafter, in our roadster, not a low one either, and at 200 never saw it.
I don't think marking the ground will do it.
There's no reason I can think of of not filling the balloons with good ol' cheap air. They'll sit on the ground. They could be marked with a dye circle for maintenance. They could be easily replaced during the day, and moved when a course is moved -- not easily done with a big dye mark, even if it could be seen.