I had the same water bottle holder on my Specialized (compact frame wouldn't allow two to fit in the normal spots) and I was able to put a small wedge under the seat in front of the water bottles. To get into it, I had to remove it, but that was fine as I didn't put anything but emergency stuff in it.

The one I used might work for you...it had a rubber gasket attachment that would probably stretch around your seat post. Unfortunately, I can't find it online at the moment. I might be remembering the make incorrectly, so I'll check when I get home and update here...

Another option might be one of those triangle shaped bags that attached to your frame. I think they make pretty small ones that might hold just enough to meet your needs but still be out of the way.