Siume,

I have a compact-frame lower end tribike that I use. I'm a small person, so I the frame doesn't have neough room to fit two water bottle cages. I've got one installed in the usual place, but for long hot rides I attached a behind-the-seat bottle holder like you. And like you, I had to find a new place to put my saddlebag.

I have a small bag, long and narrow, but I can still fit (with some fiddling) a patch kit, spare tube, tool, a couple bucks, two tire prys, and my keys. I moved it from under the saddle to the inner frame of my bike. The bag attaches to the top tube, by the saddle, and another velcro holds it to the seat tube. This works pretty well for me and b/c the bag is rather narrow, it doesn't stick out at all. I don't think this would work for some bags that are wider.

Let me know if you have any questions, I can try and find a pic of the type of bag I have.