My experience is mostly with house rabbits, who have similar dietary needs to horses. The issue is not trying to meet the energy and nutrition requirements of the animal, the issue is meeting the fiber requirements.
In fact, no animal can digest fiber on its own. Animals don’t produce the enzymes needed to break the beta bonds of polysaccharide fibers and make the nutrients within available for use. Fortunately, horses, like most other animals, have an almost invisible ally–a population of intestinal bacteria, resident in the cecum and colon, that are specially adapted to digest the fiber that horses cannot. Through a fermentation process, these gut flora produce the necessary enzymes to convert fiber to volatile fatty acids (VFAs), which the horse can absorb. Not only do the bacteria benefit (making this a truly symbiotic relationship), but the VFAs they create provide 30-70% of the horse’s total digestible energy needs.
While we often provide grain and supplemental fats to our domestic horses to give them the energy to do hard work, it’s important to remember that it’s fiber that horses were meant to use as fuel–and fiber remains the first and most important ingredient in every equine diet. It provides all the energy horses need for everyday maintenance metabolism–ordinary stuff like breathing, walking, grazing, and sleeping. Without adequate fiber, the horse’s digestive system doesn’t function properly–it loses the ability to move food particles efficiently through the gut, and its ability to conserve water and electrolytes also is compromised. Without fiber in the system, high-carbohydrate feeds tend to "pack" in the gut as well. The result is a horse at risk for dehydration, colic, and laminitis (not to mention stable vices like cribbing and wood-chewing, which often develop when a horse’s fundamental urge to chew is not satisfied). Source
If a rabbit is sick we will often provided some high food values foods like banana, but for any event lasting longer then a few hours we will also supply high fiber foods like Oxbow Critical Care which can be force feed by syringe.
If a rabbit has significant dental issues (no teeth) than canned pumpkin (plain pumpkin NOT pie filing) is a cost effective long term solution (as compared to Critical Care). We have had rabbits live several years with canned pumpkin as the primary food. Squash baby food is another alternative. You would need to check specific to horses, but most squash plants are probably appropriate grass/hay replacements.
Except you are in a drought, and it takes more water to grow a squash then it does a grass. Which leads us back around to trying to find a source of large volumes of fiber.
Again specific to rabbits you would need to do more research around horses... Clean cardboard is a favorite fiber of most rabbits. Pretty much every toilet paper tube is given to the rabbits for toys, they play with them, they chew on them and they eat them. Plain cardboard boxes without shiny print are used for hiding holes and also get eaten and need to be replaced regularly.
IF cardboard is a viable solution for Horses! You are going to need a source of high volume clean cardboard. Most of the fruit and vegetables your local grocery store gets come in cardboard boxes, which they throw out. Cardboard appropriate for transporting human food is probably going to meet or exceed any processed animal feed contamination requirements.
Again cardboard as your source of fiber for horses is speculation, wood fiber in cattle feed is not an uncommon practice. Rabbits, Cattle and Horses have similar but also very different digestive requirements.