SXSW was quite exhausting. I’m actually relieved to be back in Mountain View today. Definitely felt some of that heat!
Your best bet to get clarification from Howard Yount is to contact them directly, however, I can vouch for their tropical wool trousers being a great option in warmer months. Linen, obviously, is a good call. The nailhead trousers seem to be of the same weight of wool as the tropical wool ones, so they might work as well.
Linen would be your best bet though. Super lightweight and breezy. Downside is that they’re super wrinkly.