I have started a new project NMaven (http://nmaven.sourceforge.net) for managing .NET builds (C#, PHP, Nemerle) with Maven. The project is under Apache v 2.0. Brett P. just let me know that there is a C# plugin in the Maven sandbox; so there may be some intersection points here, where each project can benefit. The NetCompiler implementations - that I am building - compile C# (and other languages) directly using org.codehaus.plexus.util.cli.CommandLine: you may be able to leverage that directly, as opposed to using the ANT tasks for C# compiles. NMaven also has concepts of the capabilities of the build platform (architecture, OS, supported compilers, vendors (MONO/MS)) and matches the appropriate compiler based on the build requirements. These concepts may also be leveragable for the Maven C# project.
Best Regards, Shane