No the simple past tense is for the simple past, although it is a distinction which may be becoming blurred. Consider your first example, with just its first clause.
I had hoped we would leave tomorrow.
It is something you don't hope any more, though, for whatever reason. Or possibly you may have hoped it, then you were told it couldn't happen, and finally you were told that the situation had changed again. Now you can. The point is that at some point in time between then and now you had lost hope. The past predicate would still be applicable if you regained it.
I hoped we would leave tomorrow.
And you may well still hope that. Just because I was happy yesterday does not mean I am not happy today. It does not mean I am happy, either. The simple past expresses nothing concerning my current state. (Well, some things are permanently permanent -- my focus is on things that can change, like emotions.)
Now simple past with both clauses:
I hoped we would leave tomorrow, but it won't be possible.
Technically this is grammatically awkward, but it may not be wrong. You are not predicating your first clause with the meaning inherent in the second. As stated up top, this form is becoming more common as our speech becomes more, um, lazy.