is it ethical to require patients to participate in a clinical trial in order to obtain, or have a chance of obtaining, an unproven but potentially life-saving treatment?

Im not certain..

It would be unethical to use people in a developing country as research subjects in order to test a new treatment which people in that country could not afford.

It would be unethical to REQUIRE anyone to participate in a clinical trial. Such participation MUST be with the patient's consent after being told what the potential risks and benefits would be.