cdp8: you'd have to be nuts to put a bottle under the downtube! That's the filthiest part of the bike! A 3rd bottle in a jersey pocket would probably be better for a roadie who's camelbak shy.
You can't access that bottle while in motion, so when you stop to rotate the bottles, just clean it off. Orput it in a ziploc bag before putting in that cage. Then throw the bag in the trash when you rotate bottles.