Is offering services as a "Prius Handyman" a good idea?

That would be nice, but there is no such thing. Manufacturers have their own hybrid certification, for technicians already on their payroll. ASE does not have a certification program. ACDC offers Up Your Voltage for $3600 in a week-long class, about the closest thing to a hybrid certification class, but again, this is not a nationally recognized certification.

 

You might want to talk to your state's DMV or whatever state agency is responsible for regulating auto-repair businesses. If they are responsible humans they should be willing to let you know some of the key requirements and liabilities. Car dealers here are held responsible for cars and have to have insurance and a bond and you might find something similar for car repair businesses.

After talking to them, you might find that you can do at-cost or bartered work without legal liability and that could help you gain more experience and build up a network of potential clients so that when you're ready to commit to doing it professionally you have a viable business.

The Prius needs more independent technicians. I hope you don't give up on the idea.