Hmmmm.
Ok, let's say we just use Q-by-Q mode with the index enabled.
We know it's possible to start the session at a specific question (other than the first one), because that's how it works when you resume.
And it's also possible to jump directly to other questions, because this is how the index works, using javascript and document.limesurvey.move.value.
So the mechanisms are already there in the LS core.
Now if only we could read a Question_id from URL parameter and route them right to that question with the index visible, while also doing the necessary calculations to get the index and display it.
Easier said than done, but maybe some smart person could figure this out...