We had a blast with this tonight, and found our sub, but we had a similar scenario to those mentioned before in MP where at first nobody had the weapons sub menu. Later on however, it did pop up for one of us after they crashed and picked up another aircraft, and it wasn't the host. We then all tried jumping around in the slots (UH-1's) to try and get it, but that didn't have any effect.
We also noted another issue that we've seen before in a different mission (not sure which off the top of my head) where the entire radio menu is not available as soon as the players aircraft leaves the ground using the 'main' radio, but the 'intercom' radio menu works fine. When you land again, the main radio menu works again until you take off...
Gotta say again though how great this is and how far out there you must be to have dreamt up this idea, let alone script it and get it to this stage!