Sorry if this is derailing the thread too much but it's been going in this direction.
With the goal of being able to carry an additional water bottle and access it while riding, here's what the simple setup I rigged up today, using the Topeak adapter (have not taken this on the road yet):
View attachment 615
The Topeak adapter seems clamped on to the headrest tube well, after I replaced the thin stock plastic pad with a thicker one.
Some issues:
(1) The taillight is obscured from the right side. Not ideal but I don't actually ride in the dark so not a deal breaker for me at this point.
(2) The extra-large bottle just clears my helmet when I sit on the bike, after I moved the bottle cage as low as I could, without removing the pack.
(3) The pack is squashed down by the bottle/cage, still accessible but storage might be a bit compromised (and the pack cannot go any lower without altering the straps, there is one hidden underneath the seat and just above where the frame tube ends).
Those are minor issues that I might be able to clear up, but my biggest concern is:
(4) While I can easily remove the bottle from the cage while sitting on the bike, it takes quite a lot of fiddling to put it back. I can't imagine being able to do this while riding, although maybe I could learn.
Am I sort of on the right track here? Have others had issues taking out/putting back bottles mounted behind the headrest without having to stop?
EDIT: I rode with this setup yesterday. It works as a way to carry another bottle, but not to drink while on the move. I could reach back and take out the water bottle while riding, but I was holding it the wrong way to drink easily, and after a lot of fumbling I could not put the bottle back. I did not feel safe fumbling around while moving, so I'm giving up. Now thinking about something behind the seat with hoses (I've seen that in Ratz's photos) or somehow getting a bottle onto the boom (my Silvio 2.0 has no bottle mounting holes there).