Taking the OP's suggestion a step further, networking between contractors of similar talents (or put another way, friends, aquaintances etc) would allow each to 'swap' days within their projects, under the substitution clause. In business, its common to have close working relationships with what could also be considered competitors - wedding photographers do this, so that if they are ill or sick on the big day, they get another in to substitute.
Could this work? Just do a token substitution day with a mate who contracts in the same field
