There are so many wildcats it's hard to keep up with them, but I'm going to go out on a limb and say no, it is not compatible. I do believe I've seen a couple 30-223s, and the case is longer than the .300BLK.
With that said, isn't the 30-223 also known as the 7.62x39mmUSA? You could build an AR-15 for the 30-223 if you've already got the components and brass. Buying a barrel blank from DPMS (or anyone really) is easy to do, you'd just need to find someone who could chamber it for you.
Another alternative (which I think you already suggested doing) would be to see if you can re-chamber your XP-100 to .300BLK. If I recall they use basic Remington 700 actions, and there's plenty of .300BLK barrels out there - you'd just need to find a short one. I personally wouldn't use one in a barrel shorter than 9", you run the risk of it not stabilizing.