Welcome back, @Ellen_Mueller,
Can you effectively do this now with this workaround:
Multi-home the desired milestones to an adjunct project, like “ Goal Tracking Milestones” and use that project instead of the main one as the Goal Reference project.
You could even have a rule that multi-homes based on setting a dropdown “Goal Tracker?” custom field.
Hope that helps until/if this is implemented.
Thanks,
Larry