For anyone interested, I did buy a couple elastic knee supports from Wallyworld last week and used them bird hunting Saturday and a couple days this week at work. For $3 each it was worth trying. Turns out they help quite a bit, not magic but significantly less fatigue and stiffness at the end of the day.
They've been comfortable in temps to the low 60's, not sure how they'll be in summer heat. I won't have an idea on durability for awhile but for now they stay in place well. I had faced upland pants on Saturday so briars weren't an issue, but these are thick enough that they should afford a little protection as well.
I should have thought about trying these a couple years ago, definately worth taking on the trip.