After some work we are able to share some results, basically a methodology  that can be "described" and followed by more than one person. We have created a new empty repository where user interface functionality is added little by little from the old codebase and the old non-GUI calls are replaced to calls to a new, in-progress Geotools-based library.
Some of these functionalities are very easy to include, some require changes to the GT-based library and take more time. In any case, now we can estimate roughly the cost of the integration since we have been tracking the time spent and we know how many functionalities are still to migrate. Anyway, we'll see how many people is interested on it enough to give us a hand and share the effort.
We have created a project on github that contains the work in progress . As it can be seen in the developer guide , a "git clone" and a "mvn package" is enough to make it ready to run. You can find some general description of the project here .