I suppose a child could pretend - but I doubt they could keep it up, they'd forget and speak.
Ds1 was selectively mute when he was little - nursery never heard him speak, none of my friends heard him speak - he would make funny little noises and point at things he wanted. He was a chatterbox with us (apart from two dreadful weeks when he stopped speaking to us too). His nursery thought he was autistic - he had lots of indicators: lack of speech, no eye contact, no interaction with others, and he walked on his toes.
He still has little idiosyncracies which are a throwback to his selective mutism - he will always be shy, and "must speak more" is a refrain which I think will follow him throughout his life at school. He finds social interaction very difficult, even now. There was a programme about selective mutism on not long ago, with a boy the same age as ds1 - and so much of his behaviour and body language reminded me of ds1.
So, to answer your question - yes, I guess a child could be pretending, and making a game of it - but I doubt they could keep it up for long. And there are different levels, afaik - some children don't talk at all, some only don't talk in certain environments, eg., school. There was a little girl on the programme I mentioned who wouldn't talk at school or outside - but would talk at home and in the supermarket, somehow she felt 'safe' in those places.