I used to carry a frame pump on a mixte (the pump went up the seat tube between the twin laterals). The bottom of the pump was wedged into the angle between the seat tube and downtube. The top of the pump was held by a metal thingie strapped to the seat tube; it had a sort of round cap (I thought of it as a "mushroom") that held the pump. I believe this arrangement is quite old-fashioned, but it did work. I recommend a pump strap as extra security.

If you can't find a "mushroom", you might be able to use the PumpFix by KlickFix. It costs $6.50 at Wallingford Bicycle Parts.

http://www.wallbike.com/klickfix/tir...mpfix-klickfix