Is plastic wrap safe to use when tempering chocolate?
Most microwave suitable plastic wrap (cling film) still says you should avoid contact with the food and just use it as a cover. But of course in that case the food is intended to get properly hot, and of course you don't want much heat. If you do get local hot spots the film may well soften. At that point it might impart flavours to the food. It's probably only slightly more likely than getting a flavour of the plastic bowl you otherwise use. 

I write this from the point of view that you're more likely to spoil the flavour than harm yourself by slightly misusing a product that's generally accepted as food safe.