Depends on when you give them their hunter clips, and what part of the world you're in. Here in VA, the latest date to clip without affecting spring coat is mid to late January. Since you're further south, I would back that date up to late December/early January.
This is interesting to know. My furry boy was gelded mid-December. They clipped two spots on his neck to administer the drugs and he's had those funny bald patches ever since. I can hardly wait for him to shed out, grow in his spring coat, and be back to normal. Hopefully it will look normal once he's shed is winter coat.
It might depend a little on your horse as well. Last year I body clipped a horse in EARLY APRIL (last minute rated show plans, couldn't take him as a wooly mammoth!)!! I was really nervous but his summer coat looked absolutely normal. It's not quite answering your question since it was an all over cut and a trace clip but... just FYI didn't ruin his summer coat.
I'd suggest doing a hunter clip if you're going to do it this late in the season, although I doubt a trace clip would damage the summer coat, if there IS any problems, it'll be less noticeable with a hunter clip (everything but the legs and head).