According to one of my recipe books, you can soak quinoa overnight, which means that muesli ought to work:
In a bowl combine:
3 tbsps quinoa
A few sultanas / little bit of dried apricot / dried fruit of choice
Some grated apple or pear
Teaspoon or so of ground almonds if you are happy to use them
At least 6 tbsps HOT milk or formula or juice (enough fluid to cover the quinoa and other stuff).
Leave overnight in a bowl with a lid - a plastic box is ideal.
In the morning, you can loosen with a little more milk or formula and serve warm or cold.
I use oats for muesli, but quinoa *should* work just fine too. If you're using oats, the added milk/juice can be cold.
DS likes it best with milk, chopped dates and grated apple. You can make up enough for about 3 days worth in one batch, just adjust the quantities.
HTH