Sure, sea salt for kosher salt. You want something that will stick but not dissolve too fast.
I'm not sure how much of an improviser as opposed to measurer you are. The volumes of different salts don't always translate evenly. I think you might need a greater volume of Maldon than Diamond kosher. If, when you use a rub, you always think of it in terms of using the right amount of salt and don't over salt, you'll never go too far wrong.
I think this particular rub is a little bitter and too much depth for roast chicken, but there's no law saying you can't try it. If you do, for heaven's sake let me know how it worked.
A slight variation on this cocoa/espresso rub, my "Basic Beef Rub" has won a lot of barbecue and grilling competitions for a number of people competing in various beef classes; but I don't know if it's ever been used for chicken by anyone else. If so, they never told me.
My chicken rub is more herbaceous, more garlicky, and better calculated to compliment citrus. No cocoa or coffee, though.