I gave presentation to a professional group recently under similar circumstances.
My solution was to wear a sleeveless, v-neck, silk top under my linen jacket. That made the jacket much more comfortable so I could keep it on during the presentation.
I did hedge my bets, however, and tossed a light-weight, short-sleeved, silk/cotton cardigan into my briefcase. My thinking was, once I got to the venue, I could judge the temperature and substitute the sweater if I thought I'd be too warm in the jacket, or if the participants were wearing more casual attire. Since the sweater could serve as a light-weight top if I buttoned it up, I'd have a couple of options. As it turned out, the a/c was turned on high and I could have layered the sweater under the jacket and still been fine!
Another tip: After having a waiter splash salad dressing on my silk blouse during lunch, I never go to events where I'm going to be in front of an audience without having a backup top. I was lucky that time because I was able to button my jacket for the afternoon session, but it taught me a valuable lesson to always have a backup plan for my clothes as well as my presentation materials.