Maybe I am paranoid given the strength of tempered glass and you can take the following as you wish, but glass is an amorphous solid, and has some liquid qualities. Over the life of a tank (10+ years??), I am not sure if this comes into play, but can't help thinking about deformation of the glass the tank is clamped on, especially with the torque created by 15+ lbs of weight supported by a fairly long lever arm clamped to the edge of the glass of a rimless aquarium (I have an Orphek Atlantic Icon and 1/2" glass on a rimless tank), combined with all the talk of seam failures and combined with internal water pressure from powerhead. The deformation may be immediate due to the weight, or due to long-term effects. In either case I believe it is a significant enough stressor to avoid. I haven't seen any engineering write-ups on the long term effects of such stressors on an aquarium, so I passed on supporting the light from the glass of the (rimless) tank.
I chose to err on the side of caution and just hang the light(s) above the tank.