Since the front tubular was going in the trash anyway, I thought I would cut it open to inspect the damage, and to see how they are constructed.
After lots of grunting to peel it off the rim, I carefully sliced through the inner cotton casing with an X-acto knife but still managed to cut the very delicate latex tube. It appears the tube was glued along the inner surface, which would make it nearly impossible to open the casing without damaging the tube. I've heard stories of people taking off their tubulars after getting a flat, opening the casing, patching the tube, sewing the casing back together, and using the residual stickiness of the glue/tape on the rim as a "good enough" roadside repair to get them home. Unlikely with this construction.
Between the tire and the tube is some sort of very fine, high TPI polyester fabric. I assume this is the "SuperPoly 260 TPI casing" in the spec sheet for these Challenge Almanzo's.
Finally, the red latex tube. As suspected, the impact created a large pinch flat, too big for the Sealsafe to plug. I didn't check, but it's quite likely there's another identical cut on the opposite side of the tube, the typical "snake bike" typical of pinch flats.
So, that was an interesting experiment. The tubulars were fantastic to ride, incredibly supple and grippy while still fast rolling. However with the difficulty in repairing them, at home or at the roadside, means I'll probably be sticking to clinchers for normal riding for a while longer - and saving the tubulars for race day. I just did my first ride on some Michelin Pro4 Endurance 28mm clinchers, and they feel quite nice.
Not to mention, from an environmental and economic point of view, throwing away a $100 tire because of a flat doesn't make much sense.