I don't think you always have to worry about ulcers in the hind gut, instead wonder if the hind gut is acidic enough that the wrong bacteria is in there. As
said, I think I have solved my issue with my mare after years of pouring hard feed into her.
What I've been learning is that there is a different kind of bacteria needed to digest grain (oats) and to ferment hay. In an acidic environment, the acidic type bacteria thrive. Which means your oats get digested, and if you're lucky, the acid won't cause any openings in the lining of the intestine causing colic or laminitis. But it also means you don't have the right kind of bacteria to digest and ferment the hay. So a lot of the hay just mainly passes through. My mare seemed to give up on eating much hay, she'd often only eat 8 lbs a day, which made me desperate and I fed her pounds of senior feed, various hard feeds and finally oats.
Supposedly 4 lbs of oats may not cause problems. However, it might have turned the bacteria tide in the gut over to letting the more acidic types thrive. And bacteria and yeast in our body have these territorial type relationships. Different types battle against each other and take over space, and when one takes over due to the right environment, that kind tends to thrive and rule until something changes the balance again. A problem is also that some of the bacteria that live in acidic environments actually produce acid and make the environment more acidic. This is a problem in humans too, and sometimes (this is really disgusting), they have to empty a human's intestines and refill them with the content of a healthy person's intestines to get healthy again.
OK, did I have to go there? Anyway, what has helped my mare is giving a hind gut buffer (timed release sodium bicarbonate) called Equishure, but also I've been giving probiotics daily to help the non-acid bacteria take over again. It seems to be working for my mare.