If as a supply teacher you are bought in to a class and have been given no lesson plan and little time to create anything then you may worry about how and what you are going to teach the class. Using the homework ethos is a great idea when it comes to emergency lesson planning. With homework the child should be able to complete the task with little or no resources, it should be based on something that they have already covered in some detail and should be something they can do with little reinforcement.
When you enter the classroom as a supply teacher, take some time to look around at the displays within the room, have a look through the students work books to get an idea of what topics they have been working on and if there is another teacher teaching the same age group, ask them what sorts of subjects have been covered.
Depending on the age and ability of the students you may be able to set the same work for the whole class, but if you do need to set different work then use the same core idea and just adapt it accordingly.