MRP now runs on Windows using the Apache Harmony class libraries. The code is at its first complete revision and some major work is needed on signal handling and the opt compiler's stack manager, but if you've been waiting to do some metacircular hacking in Windows now here's your chance!
I got bored of waiting for DaCapo's eclipse to run, you can see from the fop performance my laptop is none too speedy. The GC warning is an MMTk feature and occurs on Linux too.
Note that you only need free tools to work with MRP in Windows, namely: cygwin, apache ant, a built version of Harmony, Visual C/Studio express edition. The debugger in Visual Studio is actually quite nice and is started in the usual way by the RVM command (ie -gdb). The feature Visual Studio has that GDB lacks is to step over call instruction in assembly code. The visualization of code inline with disassembly is also very pretty.
MRP is now running with Apache Harmony 5.0M9 or GNU Classpath 0.98. Other than keeping MRP up-to-date with bug fixes and performance improvements (although sometimes the former degrades the latter) this allows MRP to run applications such as ECJ or x10c either with Apache Harmony or GNU Classpath. Of course SPECjvm2008 still only works with Harmony.