In 7.2, it is possible to generate an implementation from the interface declaration, but it isn't possible to -- in the context of a NetBeans Platform application -- specify a different module where the class should generated.
Not quite sure if the increased complexity of the UI for the presumably more common case overweights the advantages for the less common more complex case. Especially given that creating a new class through the standard means and adding the supertype is not really difficult by any means.
I tend to agree that this would be overkill. What if the selected project does not actually have a dependency on the project defining the interface?
This situation seems similar to a "Refactor Move" operation which I would say the UI already handles sufficiently. If the target project doesn't depend on the something that the moved class depends on, it appropriately notifies the developer and it is then the developer's responsibility to handle it. Not sure who the intended user this feature is targeted for, but as a NBP dev I would find this more useful if I could at least point the generation to another project module.
Otherwise, the feature in general is "overkill" because it is trivial(because NB java editor is awesome!) to type the few key strokes for "implements" after any class and get the list of available (via dependencies) interfaces to implement.