Even simpler is to prohibit payments for these experimental treatments, but let people who whould die anyway participate in them as long as it is free for them. In return the drug companies could use the outcomes as data points towards their aproval applications.
Why would a drug manufacturer do that? Why would they offeir their advanced but as of yet unaproved drugs for free? It is true it would cost them the manufacturing cost of the drug, but what they get in return is data on how efficient/safe the drug is. Assuming the drug works well they could use that data to prove their case and get approval. And if it is safe and has the desired effect they get their aproval and they could then charge for their drug as usual.
This sets the right incentives for the drug company. If they know they are selling snake oil they would be crazy to offer it for free, especially because all it would do is to prove that it does not work. If they think it can work they gamble with their own money. Which is good because the drug company knows the most about their own drug. The patient in general is not knowledgeable enough to evaluate the chances of an experimental therapy, but the drug company is.