I think if your cold fusion idea were possible, Light would have used that idea. Part of the reason L got onto him was that he obviously needed to see the results of his experiments on prisoners, and the only way to see those results would be to have access to police records.

But if Light had the ability you’re talking about, all he’d have to do is to tell them to do it in public. A death row inmate who escaped and then drew a giant pentagram before dying would make the news. And if the victim has access to whatever inaccessible knowledge is necessary to complete the task, then escaping would be a breeze.

Light would have been dumb to overlook something like that. He was surprisingly dumb in a lot of ways, but I think that for discovering the capabilities of the death note, he’s not supposed to be dumb.