I think you'll need to use an extract to get this to work. Then you can dose it to taste before bottling/kegging.
Or you could just dry hop with El Dorado...
I don't know, the above mentioned Hell or High Watermelon has noticeable watermelon flavor and aroma and is made with watermelon juice rather than extract.
Right, but there's a huge difference between a light wheat beer, and a big Belgian with significant yeast and malt character. You may be right, but I don't know how you'd pull it off in a quad. If you add it post-boil or post-ferment, then you're going to dilute the beer down too far. If you use it for your brewing liquor I'd be afraid of boiling off all the aromatics from the juice.