I assume you mean ERU (Equine Recurrent Uvietis)?
My mare has it too! :)
The key with the flymask is that it MUST be a UV blocking mask. You are probably aware of this of course, but any old flymask is not going to protect her eyes enough.
Kensington masks and Cashel's both have some UV protection (not sure about Cashel but Kensington's have 60-something%) but not really enough, imo.
You can also buy a mask called a "Guardian Mask" which is super expensive but has a really high level of protection.
The other option is to buy UV-blocking flymask material at a fabric store (it's usually used in sunshades or to shade patios so it'll be in the utility section) and create your own mask using a darted flymask, that you take apart to use as a pattern, and limited sewing skills.
The last option is the one I did and it's been really great. I was able to create two 90% UV blocking masks, with cute trims, for under $30 for the pair. Less than one Kensington mask!
As far as supplements go, I've had luck with MSM. A Google search will lead you to the research surrounding it, but what I've read, and use, is a dose double the joint dose. To reach a level of MSM that's theraputic for eyes, you must give 30,000mg/day. It's a LOT of MSM but, at least for my mare, it really works!
I split it into 2 feedings per day.
I also, during the summer since my girl is basically in pain constantly when it's bright out - even with a UV blocking mask, feed a bit of Devil's Claw+Yucca everyday. Just enough that she's not in bad pain - usually that doesn't even end up reaching the lowest recommended dose. Those aren't great supplements super longterm but they're, imo, a bit safer than daily bute. Therefore I use just it in the summer and save bute for flare ups.
I also feed flax seed. She gets about 1/4 cup of it per day and, while I'm not sure if it directly helps her eyes, it DOES boost her immune system. Since ERU is an auto-immune disease, anything that boosts the immune system is my friend!
Apple cider vinegar may also boost the immune system but I haven't seen a whole lot of results with that. Perhaps it's the kind of ACV I buy or perhaps it's just Lacey's make-up. In any case, it might be something to try!
Hopefully that was helpful! :)