Can I use a double insulated power supply with a metal framed sculpture with LEDs
Even if you used batteries, is your tree sculpture really safe? 

If someone gets entangled in it and the structure pierces the skin and the battery voltage is, in effect, directly connected to the person (having penetrated the outer skin layer) and that person remains entangled in the sculpture I suspect that there could be serious medical implications. 

More than 99% of a body's protection against flow of current is due to the skin. A calloused, dry hand may have more than 100,000 Ω because of a thick outer layer of dead cells in the stratum corneum. The internal body resistance is about 300 Ω, being related to the wet, relatively salty tissues beneath the skin.

Information gathered from this website

I believe that before you should consider the use of an external AC power supply you should convince your self that having the tree sculpture connected to one side of the DC power is a sensible thing. I have no idea what your sculpture looks like but I'd begin by having the LEDs insulated from the metal of the tree and then start considering what sort of power supplies to use - maybe medical power supplies (that use more reliable isolation) is the way to go.

I suspect that normal "safe" power supplies do not consider that a person can be entangled in the wires.