Clone6, making a rock solid opaque white urethane is TOTALLY doable. I have done it. There's pigments you can buy that will not only turn it white, but will also make it SUPER opaque.
I have some info on my blog, which coincidentally is ALL about superman returns costume replication. Tons of info and thought process on replicating the boots, including the soles.
Might make for an interesting read for both of you.
Here's a link to the blog:
The Project Work Bench
I'm actually doing research and work on the boots right now.
Here's a link to the blog post where I solved my opacity problem, and the materials I used:
The Project Work Bench: Opacity Problem Is Solved
I have forgotten where I ordered this stuff from, but it's a smooth on product. You can probably figure it out with a call to them.
Hope that helps.