I can see how you have an extra challenge, there, and so has Donald, IR is so tricky to deal with. I'm wondering if other kinds of nuts are ok, from all the reading I've done on IR in people moderate-fat is ok and does not lead to weight gain unless lack of exercise is a problem. The Atkins diet was developed for people who have IR. Really not sure if that transfers to horses.
I wonder if one could easily make treats from soy flour and ground nut flours. Again, transferring from what I know about human nutrition, I know that many foods, if introduced regularly over a course of about 6 weeks, possibly mixed with existing rations, will not only be accepted, but over time actually preferred and could then be used as rewards. I'm thinking of all the "boring veggies" that are low-sugar, cucumber, celery, green leafies like turnip greens etc. . I used to really dislike most green veggies, until I discovered chinese cooking (my husband is Chinese-American) and learned that these can all be prepared without water but stir-fried in oil and flavored with spicy sauces.
I wonder if small amounts of berries would be ok for an IR horse (I guess only affordable if you could can find them wild or in your garden) for special treats. They have a moderate amount of sugar but lots of fiber with it.
My daughter loves to cook and bake goodies for her horse. I might make this a little research project for her.
Look what I found in the fridge this morning:
This one is not low-carb