Yep, this has to come from the inside out.
I would suggest some oil and flax in her feed. Make sure it's high quality feed and hay, amounted and correctly chosen ingridients to her level of work.
I would suggest some paprika as well to deepen her color some more, making a shine easier to show.
^That's my 27 yo thoroughbred, who hasn't had a good bath in lord knows how long, and only gets really groomed all the time during winter when she's being brushed down to put her blanket on. She's on full turnout, coastal hay, nutrena senior grain, and vegatable oil. That's it, and she stays looking like this year round. We don't really pay much attention to her, since she's been fully retired for six years now, with her source of exercise being walking around grazing, with the occasional gallops around her pasture being the crazy ex-racer that she is. So, I credit her awesome weight and shine especially at such an old age, to her nutrition. Horrible TB feet though. Not much to be done about that.. They're as good as they're gonna get.
It's all about the nutrition facts when it gets down to it.
And this is my gelding before his bath. It had been about 4-5 months without a good sudsy bath, and just hosed down after every ride.