I doubt casino lies when they say that they have 1% housr edge because this isn't really such thing to lie. But what's about provably fair, I can agree you. Also there was tagged one owner because of fake provably fair system. Slots and etc things are out of provably fair, they are programmed for profit, not for fair decisions.
P.S idea is good..
Slots
can be 'provably fair' but it's a little different. In dice, you control the odds, the amount you wager and also the direction of your range (low or high). This makes it so that even if the casino tries to manipulate the results,
any bet can be won. In slots, I would expect a hashed server seed + nonces. That way, it's one static server seed so results are essentially randomized (otherwise casino may just simply return losing outcomes given a static client seed) and you can check afterward. I'm not entirely certain on how the slot outcomes are expressed: if there is a certain combination/icon given a range or if it's simply based on 'amount won' and a visual representation is made. I do understand however that the visual aspect of slots is just that - the real results are mostly irrelevant to what you see on the screen before the final image.
Feel free to correct me if you found any mistakes and I will revise it in response. (PM would be preferred)
By the way @OP, doesn't DiceBot have an automatic provably fair verifier?