Ah ok, good that she's not currently sore. And great that you are supplementing to her hay analysis. Were you using the soybean meal to make up for a protein shortfall in the hay? Was she sound on the lucerne chaff? Feet good (tight white line, smooth walls)? If she can tolerate the lucerne chaff, there's no reason you shouldn't use it, it's just that some horses are sensitive to lucerne (alfalfa), for as-yet unknown reasons, so it's not recommended here.
I also struggle with giving my good doers a small enough feed to keep the weight off them, but large enough to even out the taste of supplements. I end up syringing half the powders.
Can you get her insulin and glucose levels checked to see if her diet is tight enough? Given the sweat marks and Scoots in your latest photos, I can see she is exercising! She is a gorgeous big gal :)
Maxine and Indy (PPID) and Dangles (PPID)
Canberra, Australia 2010