At least as it went last summer, you cannot book the Panoramic ahead of time.
And it wouldn't make much sense to, since you have to get up the Aiguille du Midi first.
The Panoramic segment is quite weather dependent. I'd check the webcams online to
be sure you're going to have decent weather before you go up. We booked the
tickets and got a departure time first thing getting off the AdM (it's not part of the pass).
Actually came back over on Panoramic again and just walked on.
You can make a reservation online for Aiguille du Midi, I believe (but can't recall precisely)
that you just make a reservation somewhere on the multipass site and then show up and
show your passes.
Although we ended up not doing this, an alternative which might be interesting is to take
the bus through the tunnel, then go up Monte Bianco, Hellbronner, and down the AdM.
Note that there are no reservations for the downward journey and you can end up standing
in line for a while, and I believe the bus is not included in the pass.