If you make a wire frame for your sculpture and pack it with sphagnum moss, then you can plant on the spagnum moss. Pick a plant with a sprawling flat growing habit like rock cress, creeping thyme, dichondra, or even some of the flat succulents like hens and chicks. What you use depends on where this sculpture eventually will be placed as all of them will need to get some water somehow and some can handle sun and others will need shade. The plants are not plastic so they will need water, the right light and some trimming to keep your sculpture recognizable. Unless your sculpture is small, most other materials will be too heavy and will crumble especially if you are putting appendages like a head and limb on it. If you can make your structure fit a more compact space like a human in a fetal position or in an egg shape then you might be able to pack something around it with a hope of it staying together, but I would still keep the core hollow because of the weight and it would be a good place to hide the drip tubing you would need to keep the plants alive.
If you want the sculpture to keep its' form I suggest you use moss for the plants.
If you make your sculpture out of terra cotta you can put some moss in a blender and break it up. Saturate the terra cotta with buttermilk or yogurt and spray on the moss solution, the moss will grow on the terra cotta. That is pretty much the technique that is done to age terra cotta pots. the moss will need to be misted every day and it likes being kept moist and shady. If your sculpture is not going to be out in full sun and you can keep it shady and moist, it should work and the moss should not obscure the features in the sculture much if the features are well etched.
https://www.bachmans.com/Garden-Care/div ... PorousPots
P.S. If you want to put the substrate in a plastic mold and leave the mold on then you can probably only grow plastic plants. Unless this is a very small piece like under 2 ft tall, you will probably have to have some kind of scaffolding inside it or the substrate will crumble if you try to move it. You will still need to have a wire framework to keep the shape. If you put in something similar to hypertuffa, once it dries, it will hold its form, but you have to get it wet again to support the plants and the two conditions are incompatible. Even a small piece will be heavy once it is wet and will not be able to be moved very well. Plants are living things and if this is to be a living sculpture you have to consider also how you are going to keep the plants alive.