Dammit, I encountered the next NPC who suffers from this bug, Ogdin Big Fist on the Isle of Despair. Sadly this dude likes me so much (I won all the pit fights, which gave me the reputation of champion), that my dread armor can't lower the reaction value enough for me to pass this... :((( Literally don't know what to do anymore. Sadly there are only magic spells to make people like you MORE not less. The developers obviously never planned for this to happen, that the value could be too high. AHHH!

PS: I'm using the quick slots now, big help!

Edit:

Alright, now I think I finally fixed this nuisance for good! I edited my savegame, so this reaction value is in an accepted range, that won't trigger this bug... Geez! Now Ogdin Big Fist lets me pass without trouble AND the cherry on top is, that I don't even have to use the dread armor anymore. Good riddance! :P Now I can finally play the game without cursing. :D

I consider this bug unsolved for most purposes, and your solution perfect. Anybody else has this problem, I'm going to suggest that solution.

I've finally done the last (I think) quest for Madam Lil (get the medaillon of beauty for her). I'm so glad this is over! Certainly will never enter this place again. :D The last time was the hardest. I had to edit my savegame 3 times before it worked without immediate attack. Never was this extreme. The reaction value changed up to 30 points, every time I interacted with anyone inside the building. Before I took the quest Madam Lil had 60, when I returned she had 20 - without me doing anything. The girls behind her ranged from 7 to 80, for no reason I could come up with.

I've just returned from the elven city Qintarra and am now looking for this book. So far I'm glad to say that there was no similar occurrence anywhere else. Hoping it stays that way. ;)

Just to kinda "close" the topic or rather give it some epilogue, I finished Arcanum! Yes, it's true, seen the ending credits and all. :P

Had to report back, because when this problem occurred for the first time, I was so frustrated that I thought I'd stop playing it altogether and now I'm completely done and even did the vast majority of side-quests. :)

All in all the game was very satisfying and worthy. It's exactly the kind of role-playing game people think of, when they say that current RPGs lack depth/somehow got "dumbed down". Countless things matter/have an effect and no playthrough should be identical, unless you really try.

Warning, minor spoilers:

My sole regret was the ending epilogues, I thought they featured lot of the things less important and forgot tons I was interested in. I was very surprised to find this half-orc prominently featured, who only played a very minor role in one short side-quest, while none of the characters present for the whole game got any coverage.
But maybe it's the same reason the game had/has so many bugs, they could hardly finish it. I don't know the (possible) story behind that at all.

Anyway, I'm gonna look back on that quite fondly. :)