Yep, that's Bolas's claw, all right. Here's the art description that went out to Karl Kopinski:
This is an abstract piece. The dragon Nicol Bolas has dreamed of the day when the shards of Alara would collide and generate massive, five-color magical power, all for him. This spell represents the realization of that dream. One way you might illustrate this is to show the claw of Bolas about to clamp down over a sphere composed of five small "comets," one per color (white, blue, black, red, green), that are orbiting each other against a cloudy gray void.
The sketch that came back was nearly perfect:
After trimming an extraneous talon from the claw and rearranging things slightly, the final version was ready: