A: When you hack (in the positive sense of the word) a compiler you have one important factor working for you: Unlike most programs, a compiler has well defined semantics. This means that you can predict its behavior when you give it a certain input. In the domain of compilers an input is source code to compile.
So, the trick for placing a breakpoint, is to use the compiler's error messages. Suppose you want to find the location where the type compatibility checking takes place. You start by writing a program that creates a type error. Then compile the program and write down the error message that you get: "Incompatible types\n Found:....\nRequired:...\n". Then use your favorite grep-like utility to find the line where this string appears and that's basically it.
The reality is a bit more complicated than that. In Sun's Javac compiler the error messages are listed in a properties file: com.sun.tools.javac.resources.compiler.properties The messages are keyed by strings that look like this: "compiler.err.prob.found.req.1". So you need to find the key in this file, then search this key inside the compiler's code base. When you do that you need to remove the prefix "compiler.err" (it is automatically added by the error reporting module).
So to conclude our little example, you actually need to search for "prob.found.req". This string has four occurrences, all inside com.sun.tools.javac.comp.Check