While many clubs and possibly all AOC holders implement a sign-to-accept policy in the tech log, the only legally required commander's signature in the tech log is post-flight listing defects (or confirming their absence).
I have heard no confirmation as to whether or not there was such a pre-flight signature in the case of the accident flight in question. There is no mention of it in the accident report, though given the report offers other reasoning to support the hypothesis that Walker was in command, it seems unlikely that such a piece of evidence was found.