While that may sound theoretically impossible, I thought what a great idea it would be to train my cat to give me a massage. While my house cat weighs no more than two pounds, it wouldn't be an issue weight-wise when it comes to her giving a massage.
Having watched all the house cats I have had in my life make biscuits while fluffing or whatever it is they do with the covers they are going to lay on, why wouldn't teaching them how to give a massage without the clawing be a good idea? Outside of the claws, unless they have been declawed, the thought of teaching a cat to give a massage sounds great in theory.
In reality, I don't think it would fly with the cats. Especially since most cats I have seen have the attention span of a gnat it seems.
The reason I thought this would be a great idea most house cats are fairly docile creatures not weighing more than a sack of potatoes. Allowing them to use their biscuit techniques giving their human counterparts a massage without getting into any trouble for their actions would make sense since their paw actions are similar to a massage.
The cats wouldn't put so much pressure onto the area needing the massage that it would become uncomfortable or painful. The cat massage would be relaxing enough to humans that the cat wouldn't feel they are in danger from their actions.
While I haven't come to a conclusion on how to train a house cat to do this, I think this type of therapy would be as beneficial to humans as animals that are trained for human companionship and other types of therapy would be.