no experience, but I remembered the cheesy ads on tv http://www.duplicolor.com/products/hottires.html
I had a friend who would buy the white "paint markers" and paint his truck tire raised letters. It looked nice and lasted pretty well. (until it got dirty)
Snow Seal white elastomeric roof coating works good to brush on wide whitewalls, it's been holding up on my '55 Dodge for about 4 years now. Would probably work on white letters too. Takes about 4 coats to really cover, and the tire needs to be really clean or it'll turn brown. I got mine at Fleet Farm, not sure if they have that in Texas.
I have a gallon of that stuff and it specifically says to not use it on rubber. Is that jut their legal disclaimer or did you have to do some prep tricks to make it stick and/or not react chemically? Or maybe it's just the California formula can't go on rubber?
Mine is Seal-Best brand, and it's holding up great. Wisconsin is one of the most lax states as to what kinds of chemicals/paint and stuff you can buy, I'd say it is very possible that what you get in Cali is a different type of stuff. I'll check my tub and see if it has the same disclaimer, I don't think it does though. I tried the Fusion and it didn't hold up, flaked off after a short while. It sticks to the rubber but it doesn't stay flexible enough for a tire sidewall.