Is it permissible to use a living person to teach Muslims how to wash a deceased person?

Methods of teaching are matters of custom, not acts of worship, and the default ruling concerning them is permissibility, unless they contain a religiously forbidden element. We do not know of anything that prohibits demonstrating the washing (ghusl (definition)) on a living person, unless this act involves revealing or touching the 'awrah (private parts), due to the Prophet's (peace be upon him) saying: "A man should not look at the 'awrah of another man, nor should a woman look at the 'awrah of another woman," and his saying: "A man should not lie with another man under one garment, nor should a woman lie with another woman under one garment."