If you're hauling water to the farm my only other thought would be to soak at home (in a cooler) and then take the hay back up with you, but that leads to a whole other set of issues so the compressed hay may be the best option.

For an IR pony anything that gets fed should be weighed and fed according to 2% of ideal weight or 1.5% of current weight (if weight loss is needed) so free feedings are not an option and particularly for a pony that's had frequent laminitis pasture is not an option either, at least not until the laminitis triggers have been addressed and removed.