Basically you use a program called OnTopReplica which replicates a part of the screen that you select and then displays that bit as a pinned window on top of all other windows. This means that you have to play the game in Full Screen Windowed mode, or just windowed, which loses a few fps but you can't have everything.
This should work fine with any game that can be played in windowed mode and doesn't support HUD moving.

