Since they are 20" wide it sounds like they are the full width of the tree, and they could cup/change flatness significantly as they dry. If you look at the end grain of the board, they will tend to cup such that the end grain rings will get flatter as they dry. (ie, they will cup away from the pith) This will be more pronounced the closer to the pith the boards were when cut from the log. Could be as much as 2" of cup over a 20" wide board. I don't know how fast honey locust dries but it seems to me that it would be particularly slow drying. Most of the movement occurs as wood loses the bound water, not so much the water contained in the cells, so you may have a good bit more movement still ahead of you. I dunno, maybe I'm tellilng you what you already know.

Your measures with slots for the fastening screws sounds good. Make sure the screws can move in the slots on their own, not just when you loosen them to avoid splitting of the top.

 

The oils you named, Graft21, penetrate into the wood and form an impermeable layer which, I think, will impede drying. However, finishes are one of my weaker areas in woodworking so my advice is to be sure you check with someone having extensive knowledge on finishing green wood.

 

The old stand by is one year per inch, so 2" thick slabs a year into drying are half way there. That said, I have dried a few big live edge slabs in my time and one thing I have noticed is that unlike thinner milled boards, heavy slabs tend to either move in the first year or they don't. Many of mine just didn't move at all or at least they didn't appear to. Don't get me wrong, I have some that moved quite a bit, they usually cup exactly as posted above. I am not saying to go ahead and build with slabs that are half way dried, but again, based in what I have seen if they don't move much the first year then they are not too terribly likely to move a lot the second. Best of luck.