Amongst professional mastering engineers the concept of using reverb isn't necessarily groundbreaking. However, for most of us, adding reverb perhaps isn't what comes to mind when we think about mastering.
Mo Volans, in his latest course for AskVideo (Reason 8 302 - Mastering Lab) explores the entire mastering process. However, in one video he examines the ways reverb can be used, albeit subtly, to help expand sections of your tracks, fill in gaps, and set some sections back.
Although shot in Propellerhead's Reason, Mo Volans video can be applied to any DAW. Here's the short video on using reverb when mastering in Reason 8:
Head over to AskVideo to watch the entire course on Reason 8's Mastering Lab.