I am the developer of Byte Buddy and I do not plan to support something like in-method byte code manipulation and here are the reasons:
For manipulating byte code within a method, Byte Buddy exposes its underlying ASM API which (to me) feels like the best way of dealing with such detailed editing. The
Advice component makes some assumptions such as an empty operand stack and overridable frames for local variables besides the method's parameters. All this makes it non-trivial to port the
Advice component to be triggered within a method rather than before and after it.
I consider to add some other component for replacing any in-method method call with a dispatcher invocation at some point (similar to Javassists limited in-method replacement) but not in the near future.