Created attachment 101810 [details]
The proposed API change including test.

@DebuggerServiceRegistrations annotation is introduced as a wrapper of several @DebuggerServiceRegistration annotations.

In @DebuggerServiceRegistration we change
String path() default "";
to
String[] path() default {""};
which should be compatible from the API users point of view.
Also we add 
int position() default Integer.MAX_VALUE;

Similarly we change "String path()" to "String[] path()" in ActionsProvider.Registration and ColumnModelRegistration.

Y01 Don't change type of an attribute.

Changing type of attribute is source compatible, but code like:
String value = annotation.path();
would no longer compiler and link. So the change is not binary compatible. I don't think you need this change anyway. Instead of
@DebuggerServiceRegistrations({
    @DebuggerServiceRegistration(path="unittest/annotated1",
    types=TreeModel.class),
    @DebuggerServiceRegistration(path={"unittest/annotated2", "unittest/annotated3"},
    types={NodeModel.class, TableModel.class})
})
you can write:
@DebuggerServiceRegistrations({
    @DebuggerServiceRegistration(path="unittest/annotated1",
    types=TreeModel.class),
    @DebuggerServiceRegistration(path="unittest/annotated3",
    types={NodeModel.class, TableModel.class}),
    @DebuggerServiceRegistration(path="unittest/annotated2",
    types={NodeModel.class, TableModel.class})
})

Y01: Using @DebuggerServiceRegistrations for multiple paths is possible,
     though not very practical, mainly in case of
     ActionsProvider.Registration.path() and ColumnModelRegistration.path()
     we'd need also ActionsProvider.Registrations and ColumnModelRegistrations.

I do not think that binary compatibility is a real problem in annotations that are processed by compiler. IMHO calling annotation.path(); in a 3rd party code is really a corner case.

But if we insist on binary compatibility in this case, I can think of adding something like:

/**
 * Optional paths relative to {@link #path()} to register this implementation in.
 * Usually the session ID, view name, etc.
 * @since 1.28
 */
String[] relativePaths() default {};

In fact path() itself is relative to "/Debugger" path already. Thus relativePaths() would be relative to "/Debugger"+path()

The usage of special relpath is acceptable solution. Its only drawback is that it would be unique for debugger. I'd like to re-advocate @SomeAnnotations (with s at the end). That is how we generally solve the issues of multiple annotations on an element - thus users will see consistency and consistency is good. We already have:
http://bits.netbeans.org/dev/javadoc/org-netbeans-modules-editor-lib2/org/netbeans/api/editor/EditorActionRegistrations.html
http://bits.netbeans.org/dev/javadoc/org-openide-util-lookup/org/openide/util/lookup/ServiceProviders.html
http://bits.netbeans.org/dev/javadoc/org-openide-awt/org/openide/awt/ActionReferences.html
I think it would be consistent and beneficial to have also DebuggerServiceRegistrations, ActionsProvider.Registrations and ColumnModelRegistrations.

Integrated into 'main-golden', will be available in build *201009070000* on http://bits.netbeans.org/dev/nightly/ (upload may still be in progress)
Changeset: http://hg.netbeans.org/main/rev/04d674b90c8a
User: mentlicher@netbeans.org
Log: #190080 Adapt views to various extensions of TreeModel and NodeModel, for correct lookup result.

Created attachment 101915 [details]
Modified API change that keeps binary compatibility of annotations.

Y01) Thanks for your comments, I've attached modified API change that introduces the compound annotations.

Integrated into 'main-golden', will be available in build *201009080000* on http://bits.netbeans.org/dev/nightly/ (upload may still be in progress)
Changeset: http://hg.netbeans.org/main/rev/04d674b90c8a
User: mentlicher@netbeans.org
Log: #190080 Adapt views to various extensions of TreeModel and NodeModel, for correct lookup result.

Thanks for the review, I'll push that change tomorrow.

Pushed as changeset:   177346:56b812ba092b
http://hg.netbeans.org/main/rev/56b812ba092b

Integrated into 'main-golden', will be available in build *201009110000* on http://bits.netbeans.org/dev/nightly/ (upload may still be in progress)
Changeset: http://hg.netbeans.org/main/rev/56b812ba092b
User: mentlicher@netbeans.org
Log: #190080 Allow multiple debugger service registrations and position order.