Tonight I demolded the belt I poured up last night, and it turned out really great. It still has some flaws, but they are the type of things that repeated attempts will surely fix. Most of all, the surface the belt was sitting on was not completely flat, so the goop all built up on one side of the mold, leaving the other side rather anemic. But no big deal at all.
Here's a close up of the buckle. You can probably see some air bubbles, but notice the color and opacity. Opacity is perfect, color is close. Still some work to do on color matching, but I really feel like I've made a huge leap in the right direction.
Here's the full belt. Looks pretty good.
This close up of the strap really reveals how opaque the belt turned out to be. I consider that problem licked. Notice that you cannot see the belt webbing inside at all. Very nice.
So the lessons learned from this are that I need to get some white pigment to lighten it up a bit, I need to make sure the surface beneath the mold is level before pouring, I need to de-gas the urethene before pouring, and I will continue to use a mold release. Oh, and I also need to put some webbing reinforcement into the belt, right around where the faux belt loops will go. The urethene gets very thin right around there, so I'll have to watch it.