You decided to put the question in a general way and finally revealed the 100-200 mV requirement. If you intend a technical solution for a real world problem, this changes a lot.
As a first remark, it's strictly spoken not understandable, why you focus on switched capacitor and exclude inductive converters. At least, you didn't tell a technical reason. Either, it's a unfounded preliminary decision, or you have more unsaid requirements.
Secondly, step-up converters with input voltages down to 300 mV are commercially available. They possibly don't fit your requirements but at least tell about feasibility. Lower input voltages most likely require different circuit concepts. But, the problem should be discussed in terms of actual required voltage and current.