AFAIK, lazily loaded (or pluggable) modules can't be done in GWT. Partly this is due to the way the GWT compiler works - it likes to import all the code that it is ever going to see and then optimises and prunes it as viciously as possible (to make the resulting JS files as small and lead as possible). If it doesn't have access to all the source code up front, it might make optimisations that will break the pluggable modules (especially since nothing in the "core" application may reference the classes that the lazily-loaded modules need to work properly - the GWT compiler would prune those "unused" classes in the core module).
This stackoverflow question from May 09 asks the same thing and has the same answer - it can't be done.
I searched high and low about six months ago for an answer to this problem, because I really wanted to do what I believe you're asking for. Never found a solution.
(I haven't used GWT 2.0 - it might have addressed the issue)