I dislike walls in general and they are not practical when there are thousands of feet of OB to mark. Flags work fine for people actually in attendance. I think the "problem" will eventually be solved on the video production side with sone sort of overlays.
I think that you may be underestimating the difficulty, cost and practicality of solving this on the video production side in a live setting. Eventually, assuming the sport gets ongoing, mainstream, broadcast coverage, I agree that this type video effects could be made to happen. But I'm not sure it gets to that point if the non high-tech video coverage is too confusing for the uber-casual viewer.
It will be really interesting to see what the post-production of the DDO looks like when it gets to CBS.