I tried this already.
It appears to be a simple warp hack at first, as you said the data is already there. All they did is change what room the door goes to. Anyway, I did that and was able to enter the slots room just fine (the text wasn't translated, but w/e). But, when I went to actually play the slots it brought up the voltorb flip game. =/
After that I was poking around in the ROM at bit more and noticed that the graphics for the slot machines had actually been replaced with just solid color images. This wouldn't be that surprising if you didn't know GameFreak's habits, but I've been doing this for a while and I can say that they are very lazy about removing anything. There is still random data and graphics from platinum just sitting in HG/SS unused. The fact that they actually "removed" the slots graphics says a lot. Most likely they wanted to play it safe and avoid another GTA-style scandal. Point of this rambling paragraph: I think the code that works the slots was physically removed. If that's the case, it would require a very involved assembly hack to bring the slots back to the english version.
So, I guess I'd have to second the guy who said to use the translated Japanese ROM.