I don't know how good Shaq's takedown defense is. That would be the key, Lesnar's game so far has been predicated on taking people down and working a savage ground game. We have yet to see if anyone can stand and trade with him, really.
But no, I don't think Shaq could do much against Brock, simply because he's old. He'd gas even if he made it out of the first two minutes.