A few examples:
In "A Tale of Two Cities" there is a scene where a man is just waiting boredly for his breakfast to be served to him. He is described as looking "remarkably like someone sitting to have their portrait painted."
So you can definitely try to describe the expression with a comparison people are likely to understand, that still won't over state it.
If you do actually want to describe what it looks like directly, without making it feel overdone, try an inherently vague description, that would only give your readers a hazy idea of the expression, thus keeping it from looking overdone.
For example: Sat studying the page as if he was listening to the paper.
Or for one from my story:
I sat up, resting my weight on my elbow so I could see his face, but there was nothing readable in the line of his mouth, his dark eyebrows.
*
The thing is, that even when people are doing normal things, we'll generally have some sort of expression, even if it's 'blankness' (yes, blank counts as an expression.) So if you want it to seem mild, intentionally, I think the options above are your best bet.
Hope that helps you!