Hibiscus is a safe plant to use - in fact the leaves are supposedly quite nutritious and healthy - but it is very difficult to grow in chameleon cages because it needs a lot of light. The lights in a chameleon cage are usually not strong enough for hibiscus (and if you put lights in that are strong enough, they would probably be too strong for the chameleon).
One way to deal with this problem is to buy two (or more) plants, and to rotate them in and out of the cage (i.e. a month in the cage, then swap it for one of the other plants and allow it to recuperate outside in the sunshine).
It's not an ideal solution though, because I think it disrupts the cham (they can get very territorial, and also they get used to the layout of their cage - the tree you remove might be the tree that has their favourite sleeping perch in it).