Can I combine step-up and step-down designs for switched DC-DC convertor?
For your application exists a different category of DC/DC converters called buck-boost converter which handles exactly what you are describing.

They reach efficiency levels of both designs, mostly dipping if the input is close to the output voltage.

They are available from different manufacturers, just like you'd expect.

A word of warning for your special application (I tried something similar and killed my MP3 player with it): in case of no load the voltage of your dynamo will get very high, use some sort of voltage limiting at the input of your buck-boost converter.