First, having the Conditional attribute is not equivalent to having #if inside the method. Consider:
ShowDebugString(MethodThatTakesAges());
With the real behaviour of ConditionalAttribute, MethodThatTakesAges doesn’t get called – the entire call including argument evaluation is removed from the compiler.
Of course the other point is that it depends on the compile-time preprocessor symbols at the compile time of the caller, not of the method 🙂
But no, I don’t believe there’s anything which does what you want here. I’ve just checked the C# spec section which deals with conditional methods and conditional attribute classes, and there’s nothing in there suggesting there’s any such mechanism.