Dear Gilli,
Please do not take what I am going to say personally! I respect you as a fishkeeper, and I normally agree 100% with the advice you give. I am only disagreeing with your opinion on this one thing, not you!
Of course sand is viable in your tank. Where do you think all the 'gross' poop is going now? Just because you can't see it doesn't mean it isn't there; it's just sinking in down through the gaps in your gravel to fester.
What looks 'gross' to you shouldn't matter when it's the happiness and welfare of your fish at stake, and all the cory 'experts' I've ever seen or heard, plus almost every cory owner, agrees that they really do need sand to be 'happy', as far as we can judge what 'happiness' means for a fish, which must surely include being able to express their natural behaviour.
You could always just put in one area of sand; I have a mix of gravel and sandy patches in my tanks and it works fine. I think once you've seen cories digging in sand you'll understand what we mean; I used to keep my cories on gravel too, and I honestly thought they were fine; it's only when you see the difference that you realise that a pure gravel substrate gravel just isn't suitable for them.