Ditto on using the tri bike.

Do you know if there's an aid station in the middle of the bike? If so- just start with a bottle you can throw away- dump it when you get to the aid station and pick up a new bottle. (Or if they're only giving out twist top bottles use it to refill your bike bottle.)

Another thought would be rearranging the computer set-up so you could add an aero bottle (it might take some fiddling but there are all kinds of mounts).

Good luck!