30 -45 minutes isn't long enough to get really bogged down in specific gear items. Is there any possibility that you could make this a multi-week project:

Week 1: have the boys bring their loaded pack to the meeting before your talk, where the Scoutmaster can weigh them and make a record.

Week 2: give your talk, and issue a challenge to the boys to lighten their packs.

Week 3 or 4: have the boys bring their loaded packs to another meeting, weigh them again, and record the new weight.

The night before, or morning of, the next hike: do a shakedown.

I haven't figured out how you handle the problem of cheating in weeks 1 and 3 - there's always the chance they would simply leave something out (like a tent) just to get a lower weight.

As we all know, "lighten up" is a habit we acquire over time; one talk alone may not get the boys into the habit the way building it into the program might.