I don't think this is possible...But there are smarter people than me developing PowerApps.
I'd like to store the name of a context variable in a context variable so that I can pass it via Navigate(). The idea being that a screen may be called from different places and depending on where it is called from a different context variable may need to be updated.
The way I'm working arounf this at present is to pass a context variable and then evaluating that with a switch() statement to figure out which variable needs to be updated. This means that I have to manage change in two places. The calling screen and the called screen. It would be nice if there was an evaluate() function that I could use in an UpdateContext() statement. e.g.