Well...there's some good, there's some bad, and there's some ugly, which is kinda what I expected. It looks like it'd be simple - just pour concrete in the form - but there are some very critical things that have to be just right for everything to work, and it's rare to be lucky enough to nail those things on a 1st try.
The good part was the form. It held up just fine and, with a few tweaks to how I go about actually constructing it, I can replicate that process whenever I cast the corbels for real.
The bad part was not vibrating the concrete enough after it had been placed. See all those little holes? Those were air bubbles, which is what vibrating is supposed eliminate. Obviously, I didn't do a very good job - either in method or duration - vibrating the concrete.
And then there's the ugly, which I sorta saw coming. The concrete mix I used was all wrong, too much relatively large aggregate and not enough fine aggregate. In other words, too much gravel, not enough sand. Next time around, I think I'll go with 2 parts sand, 1 part cement, a tiny amount of plasticizer, and then however much water I need to get a decent concrete flow.
So...round 2 will be next weekend, hopefully with better results.