Download and install the latest version of Mono (2.4.3)
From C:\Program Files\Mono-18.104.22.168\bin, grab these files: mono.exe, mono.dll, libglib-2.0-0.dll, libgmodule-2.0-0.dll
From C:\Program Files\Mono-22.214.171.124\lib\mono\2.0, grab these files: gmcs.exe, gmcs.exe.config, mscorlib.dll
Copy all these files to Unity\Editor\Data\MonoCompiler.framework
I'm guessing that this works because the C# 3.0 compiler still compiles to bytecode which is compatible with the mono runtime used by Unity 2.6.
I might try this out later, and post instructions for OSX.