In reality, RAM doesn't so much instantly lose its contents on power off anyway, the freezing just slows it down. IIRC there are cases of recovering significant portions of RAM a half hour or more after poweroff even without freezing.
You're right, I think an attack could be possible on some computers if the agents act very quickly. I don't think 30 minutes would be possible though, judging from what is said and shown in the original cold boot video: