Currently, the Find Usages dialog allows you to set scope to "Open Projects" or "Current Project" (plus some more scopes).
It would be useful to have a scope for "Open projects and their dependencies" and for "Current Project and its dependencies". These scopes would look not only in the open project(s), but also in projects this project depends on, which are not necessarily open in the IDE.
I believe the scanning infrastructure has the data for these dependencies already, so it would not create a performance problem.
Please also consider to make the fix working for official OpenJDK freeform projects as described in bug 185726, e.g.
Duplicate of https://netbeans.org/bugzilla/show_bug.cgi?id=55291 ?