My current vet is Dr. Diane Poliakof in Elmwood Park. I have been going to her for about 3 or so years, and I cannot say enough good things about her. She is very honest about where she has seen conflicting research, gives her recommendation based on her experience and perspective, and is always non-judgemental on my decisions (whether they align with her opinion or not) in the process.
She is not doing active research on this. I had asked her about this at one point (we were discussing a trend she was seeing between teeth issues and stones), and she pointed out that publishable research involves a very negative impact on your control groups of animals. She is somehow affiliated with a university and often has students shadowing her, she is very up to date on other publications and current thinking, she sees guinea pigs on a daily basis, but she does not conduct specific animal studies. (I also do not think she would ever recommend the oatmeal.)
Her recommendation is 1/4 green pepper, unlimited greens, unlimited hay. And then a few bits of other veggies in a variety, but not at the expense of the pepper and greens. There are a lot she eliminates as well, such as anything that is potentially gassy she does not recommend even in small amounts.
Prior to her, I was seeing Dr. Byron de Navarre in the city. Although he may be involved in some research, I again don't think he'd recommend a diet that contained oatmeal. (To anyone looking for a vet, he, too, is wonderful and highly recommended.)
To those that are hung up on needing to feed pellets, I feel like that is akin to us eating prepared and processed foods that are vitamin fortified. I think they are a convenience because they package a recommended percentage into an easy to feed and easy to store bulk form. Yes, my pigs still get some, but I feel like KMS is probably the best pellet out there, and I give it as a treat, and cautiously because of the calcium issues. I also had an overweight sow and underweight neutered male pair that both regulated back to better weights when the pellets came out of their diets. I had been hesitant to eliminate the pellets because the male always was always at a low weight, but decided to give it a try and was pleasantly surprised.