Certainly make sure that health wise she is 100%. She should be a good weight, teeth in good condition etc. And yes I agree that oil will help.
Just bare in mind that long and low is very, very hard work for a horse with no muscle and little fitness - if ridden correctly. If you allow the horse to plop along on its front legs with it's nose dragging on the ground you're achieving very little except perhaps stretch the horse's neck a bit. A true long and low needs to be ridden with active hind legs and a swinging back, and the horse off the forehand. Physically, this is very difficult and a big reason in why we don't ride young horses in long and low for a long time - until they have built the muscle and balance to hold this position for short periods of time.
Right now, I'd be more focussed on getting the hind legs motoring, than putting her nose on the ground. Keep the head and neck up - not hollow, but in a higher position, around level with the wither, and focus on driving her with those hind legs. Get her to release her back by riding forwards and giving her the opportunity to swing. A swinging back is a strong back - she will build topline quicker doing this, and feeding her any kind of fancy, expensive gimicky supplement.