JRiver Media Center, as of version 19 supports HiDPI/retina displays (more or less anyway).
The problem with the current implementation (in my opinion) is that there appears to be no way to use separate images for when you're using the scaling functionality, and when you're not. This results in skins with support for HiDPI looking somewhat blurry on regular monitors, even when you're not using the scaling function.
It would have been really nice to have the possibility of using separate images instead, similar to how OSX/iOS handles this. That way images would be razor sharp, no matter what monitor you're using.
What this means for my skins
For now I've decided to just not deal with the problem at all, and instead just support what most of you are using; regular monitors. At some point or another, with 4k displays becoming more and more common this will have to change. Right now I'm of a mind to create separate hiDPI versions of my skins instead of degrading the user experience for most users, even if it means more work. Decisions, decisions, decisions...