Have you considered the WR50? They are 30mm wide and 50mm deep. If the 105% "rule" is really important to you, that would still allow you to ride a 28mm tire. I'm actually currently riding 34mm tires on Lightbicycle rims that are only 28 or 29mm wide and it never occurs to me during a ride to wonder how much faster I'd be if only I'd stuck to the 105% rule. Fact is a fatter tire will change your ride way more than the couple watts you'll save by sticking right at the 105% rule. Especially for a long endurance ride, if you haven't tried fatter tires I'd say it will rock your world if you ever do.

If we assume that a tire/rim combo that sticks to the 105% rule will save you a couple watts due to aerodynamic benefits over one that doesn't, you still have to ask yourself how many watts you'd save riding a wider tire due to reducing suspension losses. It might just be a wash, and you're still left riding a much more comfortable tire. I rode 23mm tires and noticed a benefit going to 25mm, then noticed another benefit going to 28mm, and now these 34mm tires are the most comfortable I've ever ridden, and on my 46mm deep 28-29mm wide carbon rims I doubt I've given up a thing that I'd ever notice in terms of performance. Given there's an option that would both give you wider, more comfortable tires, and also stick to the 105% rule since that seems to be important to you, I'd say you've got a perfect solution in front of you.