I doubt that such a mix would happen naturally. I also cannot give you a reply without a plea to please not try such a hybridization. The world does not need more designer fish IMHO... there are already plenty.
I mean the fh is already a hybrid. Never kept uaru but a male fh is very aggressive. At least the ones I have kept and seen. It would never happen naturally due to flowerhorn not being in the wild. The world doesn't need these fish but you might find an interest in them. The purists always look down on these things (Not labeling anyone). If you are successful just don't sell or give away the fry. If you do, make sure you let the people know they are hybrids and not to reproduce or distribute. Good luck.