Has anyone written about platonism with regard to emotions? I know that
There have been a number of philosophers in the Platonic tradition who have attended to sadness and the whole order of emotions in terms of proper pleasure and pain. You can find the latter in Aristotle, and more explicitly in Augustine's idea that there is an ideal order of love (ordo amoris), proper things we should feel delight or sadness in. In the modern era, one of the more fascinating philosophers to think systematically about values in the Platonic tradition (but he is no commentator on Plato, he is working out a novel ethic) is Max Scheler (1874-1928). You might find his book Formalism in Ethics and the Non-Formal Ethics of Value fascinating.