I would decrease the strider and add a vitamin supplement. If you want easy, go with a smart pak. I'd get Smart Vite Maintenance Grass, Cool Calories (solid fats), and Smart Digest. The maintenance grass is all the vitamins and minerals she needs. The weight builder will add fat without hyper calories. The smart digest is a pro/pre-biotic to make sure she's properly digesting all of her food (optional).
You total SmartPak bill would be $43 per month.
12/22/2008 Cool Calories 100 $12.95
12/22/2008 SmartDigest $11.25
12/22/2008 SmartVite Maintenance Grass $19.10
The SmartVite is made for horses not getting any (or enough) fortified grain. It will 100% complete her diet. You need some kind of supplement that is specifically for your situation, no or not enough fortified feed. A general vitamin/mineral supplement isn't going to cut it. Other supplements that would be good include Select II, Linpro, Uckele Equi-Base grass, and Augment. I'm sure there are others, those are just the more common ones.
If they won't increase her hay, then buy some hay pellets, preferably Alfalfa/Bermuda mix or Timothy mixed. Have them feed her 1 to 2 big 2qt feed scoops with each serving of hay, in addition to her grain and the above vitamins.
The mina-vite is definitely a good option instead of the grain. I would add some hay pellets to that, for extra calories.