Are you speaking of the lilac shrubs that are usually planted out in the garden? (they do get huge) Don't know how well a small shrub would do in a chameleon enclosure, I know they need pruning and they would still need fertilizer which you surely would not want to expose a cham to.
Are they poisonous? Not that I am aware of as the flowers can and are eaten by humans.
But as I said, I don't know how they would survive in that sort of environment over time..