I love tribute's essential K as a ration balancer. Sometimes I have to use Purina Enrich due to supply issues where I live, but I really prefer the essential K.
Can you add oil to her feed? A cheap and dense way to add more calories - won't increase the bulk too much. You can start with just a little and gradually increase. Most horses can tolerate up to 1 cup poured over a meal.
What about offering her "snacks" of alfalfa pellets or beet pulp as you tack up/groom and after the ride? I know some folks will say not to feed anything before you ride, but check out research on this topic from the endurance world - probably not an issue unless your rides consist of REALLY heavy work. That way you could split her feedings into even smaller amounts.