You want to know if you should use Trailer Tires in a trailer application? ST rated tires are made specifically for trailer use. Why are you worried about the speed ratings of the tires? How far over the speed limit do you really wanna tow?
Myself, I'd use bias ST rated tires. Radials don't really help a trailer, different load forces than in wheel-driven vehicles. Plus, (devil's advocate voice) ... *IF* you were involved in an accident, the other side's creepy ambulance chasing lawyer could say the use of tires not rated for trailer usage contributed to your fault in the accident, whether it's true or not. It's all about what you can't make people believe in court. Yeah, sucks, but that's the litigious reality of our society today.
You have to start somewhere to get to the middle of nowhere.