I dont know anything about the tire making process (wish they would put it on How Its Made)
I do know they are made from molds but thats about it so I havent understood why white letter tires arent offered anymore in the more current sizes.
I can see that on may of the 40 and 30 series tire how there isnt much sidewall to work with but many of the 16 and 17in tires have raised black letters so it seems like it could be doable.

Then again its more likely that I'm just being my oddball self and looking for something that just isnt mainstream enough...oh well,

Vive' La Difference!