My GM has run into this issue repeatedly when running outdoor encounters, and I volunteered to post this for him. Basically, any time there are things like cliffs where a character at the top of it can see down, but not vise versa, there's no way to represent it. The easiest solution would to be add some kind of one-way wall to the dynamic lighting system. I can't see it being the most frequently used thing, but seeing as we've yet to find any sort of workaround, I think it would be a really good addition, and add a little bit more versatility to the dynamic lighting (easily roll20's best feature)