It helps to remember that Quartz is Apple's answer to the (very) expensive Display Postscript rendering engine by Adobe. When NeXT and Sun made their application environment specifications (OpenStep) open source a new project was formed by the open source community call GNUstep (a window manager/application environment for unix and unix-like operating systems). Being an open source project, they also needed a way around Adobe's Display Postscript. The solution they are working on is call Display Ghostscript, but I'm really not sure just how far they have gotten with it. Everyone on the project does this in their spare time.
Also Aqua and Quartz are not open source. These are proprietary technologies owned by Apple. The only people who would be working on a port would be them, and they wouldn't let people on the outside know about it (or have access to it).
One idea that was floated around for a while was replacing Mach OS in Rhapsody 5.1 with Darwin, but as it turns out Darwin is to different for this to work correctly (and generally there would be little noticeable improvement even if it did work).
If you are looking for a theme Im quite sure that some one has made an Aqua theme to run in WindowMaker or the like. I havent been following the themes for quite some time so Ill leave that to some one else to answer and provide more information.