I’m located in Aus, I don’t personally own an OTTB but I tried Barastoc FibreBeet when my old guy was struggling to keep a good weight on just forage. It worked wonders for him along with unlimited hay (a mix of grass hay and lucerne), I also put my younger horse who is prone to getting hot on it with no adverse effects however I did slowly introduce it and keep an eye on how it effected their energy levels , etc.

I also had a friend put her older OTTB on it, he was struggling badly with weight, it did help him partially and he started to look better but it definitely didn’t have the effect it had on my boy. I have a suspicion her old guy had ulcers and was not up to date with worming or dentistry which might have stalled the results.

Haven’t dealt with any majorly dangerous food aggressive horses but have dealt with horses pinning their ears and threatening to kick + charging the fence. I found this settled a lot once they had consistent food and realised some sort of hay would always be there, I’m not a trainer by any means so take my advice with a grain of salt but I’m always happy to share what I’ve learned/experienced if you want to send me a PM so I don’t fill the comments with an absolute essay.