Good question ;)
I'm afraid Eclipse WTP's EAR classpath container does not allow this :(
There is though a workaround, namely when it asks for the source you sohuld be able to add the source to the servers launch configuration.
Let me know if that works for you.
I debuged in eclipse less 5 times, it's so tiring!
I always use logs.
what is so tiring ? it just works...