Yes, alternatively researching soil analysis of your area/ area hay was grown would give you a bit of an idea of vitamins/minerals that are high and low in your area. Like Acadianartist, my area has soil analysis of high iron, which also means low copper, and low vit E and selenium. It doesn't give me an entire picture of my horse's nutrition needs, but It gives some indication of what I need to look for (low/no iron, sufficient copper and supp. vit E). Soil analysis should be available online for your area.
If possible, I'd also suggest pairing that with a blood assessment to have a better picture. I do find that blood assessments are not entirely accurate on their own because they vary throughout the day and can get influenced by other factors. For example, true iron deficiency (from decreased intake/absorption or external blood loss) is a less common cause of low serum or plasma iron concentrations. Some drugs can cause iron to appear low, time of day, copper deficiency (an essential cofactor of hephaestin, which permits release of iron from intestinal cells), zinc excess (inhibits copper uptake) being a few examples. So, it is preferable to have environmental values as well in order to get a broader picture of what is actually needed. Keep in mind that hay analysis can vary too.
Also, like Acadianartist, I can also find no ration balancers in my area that fully suite the nutritional values in my area. Most, if not all are extremely high in iron and others with low/ no iron are deficient in a number of other nutrients needed. I ended up going with a vit/min supplement (Horsetech, who also does custom) and soaked timothy/alfalfa cubes to carry it. You will need something with a little more substance. I'd suggest finding out what is needed in terms of minerals/vitamins lacking and overly abundant, then structuring your horse's diet around that, which may take a bit of work and learning, but highly worth it in the end.
Feed XL is a good online (and free) program that has many feeds on file and helps you balance your horse's diet. More roughage is generally the best idea (and easiest to digest) for putting weight on. Roughage such as more hay, timothy/alfalfa cube mix (better Ca:P ratio), beet pulp are good ideas) and cool calories (oils- mainly omega 3s would be beneficial) would help keep weight, but all should be balanced.