Legally - I don't think there is a helmet law in Texas, is there? So no worries there.
Safely - Correct me if I'm wrong, but isn't the major/only difference between SA and M helmets is that the SAs need to have a certain lining material that is fire resistant? I remember something about multiple impacts when I was looking at helmets for my snowmobiles too, but don't remember seeing any concrete proof of that.
That said, I always had M-rated helmets for my bike (and snowmobiles), but I'd MUCH sooner wear a current SA helmet than none at all.