Is it possible to use .NET Reflector (or an other tool) to modify and recompile the code quickly (that is, without dumping the source and then use Visual Studio to recompile it)?
You can probably use the Reflexil add-in for Reflector to do that:
Reflexil is an assembly editor and runs as a plug-in for Reflector. Using Mono.Cecil, Reflexil is able to manipulate IL code and save the modified assemblies to disk. Reflexil also supports 'on the fly' C# and VB.NET code injection.