I don't know if this would work, but would it be possible to find someone who has a cat with a litter of kittens and just smuggle this one in? A mom cat has such strong instincts that she might accept an addition. The great thing is that while they are maternal, they can't count.

If this is possible, maybe you could get a towel and rub it on the litter and then on the orphan - that way the mom will think that it smells like her until it is too late.