You don't have to be registered where you actually live all the time. Most homeless register at one of the homeless shelters, where there is usually someone who handles letters. Homeless do exist, sadly.

Thats the only thing you need is place where official communication with you can happen. In practice, that means that there is some way to deliver letters for you.

Houses without address are pretty hard to find in germany. Building enforcement is very strict and every piece of land is owned and usually has an address before a house is built.