Is there a way to display audio bitrate in Finder? I want to have a quick and easy way to check bitrate of compressed audio files, without having to open iTunes/Quicktime etc.

Either a Finder option (Get info/Inspector/List view) or perhaps an AppleScript via Right click > Service would do, via output to Terminal or system pop-up.

Are there any established solutions out there, or will we have to look at a scripting solution?
You could fashion a service in Automator using the Terminal.app and your shell with something like:

mdls -name kMDItemTotalBitRate file