Interesting question.
Maybe the simplest way to get it all in the same view is record employee Leaves as special Project Tasks: you would have a "dummy" Project for "Leaves", and have a Task for each each leave.
Now, do you have a good reason to track Leaves in the HR module?
If not, problem solved.
If you do, then you could have some mechanism to keep HR Leaves in sync with Leave Tasks.
The simplest way is to do it manually.
If this is not viable, then you could create an Automated Action on HR Leaves, that would write on Project Tasks any changes made, on even a small module that adds this mechanism to HR Leaves ORM methods.
This would require some Python skills, though.
If you're not comfortable with, and it's important enough, why might consider hiring a coder to do it for you.