you can of course use the eclipse compiler as well, it is available as separate download. Or javac should be possible too. I just mentioned janino, because it most probably does what you need.
If it is stated that Groovy compiles straight to Java bytecode, it does mean that there is no interpreter working on some kind of AST. Ever thought of enums without the java.lang.Enum base class? Even if it is bytecode, it does not mean it is independent of other classes.
Extending GroovyObject or not makes a big difference when it comes to methods and properties.
If I understand you right, then you want to have the ability to write groovy code without the class being a GroovyObject. But then you still need the library, because all the neat things Groovy is doing are inside this 1MB big runtime lib. You can loose that lib only if you are not doing anything that Groovy does, or by inlining... in the end it does not matter, you get just a subset of Groovy, not really what Groovy makes groovy.
Even if Groovy will somewhere down the road get a static mode of some kind the runtime will most probably still be needed, because not all functions would be inlined then.
Groovy is able to interact with java quite nice, so you have always the possibility to create beans in Java and use them from Groovy. You can even create these Beans at runtime, can hook in the runtime compilation process and other things... But all in all it makes no sense without the Groovy lib.