Our cat does the same thing, but she will go on the bean bag (probably because it moves like litter) or if a plastic bag is left lying around she will use that - she is the same and only does this when the litter box needs changing.
Its almost like she is saying "I'll teach you not to change my box".
The annoying things is she will sit there and watch me change the litter, as soon as it is done and I step back she hops in a squeezes a bit out - does it everytime
I think bicarbonate of soda is good for getting smells out of fabric, although I've not tried it. I use Febreze too.