Here it is: http://code.google.com/p/ductilesharp/
I have created the "Mono C# extensibility project" with the intention of making it straightforward to write custom add-ins for the Mono C# compiler.
Here's a link: http://github.com/rcook/mono-extensibility/
Someone asked me the other day how I'd go about instrumenting the IL generated by the Mono C# compiler: something to do with custom profiling I think. I still haven't come up with a totally satisfactory answer but here's a patch of some work I did showing how I hacked away at various bits of the sources to emit custom method prologues and epilogues. Download the patch and apply it to your Mono Git repo.