I managed to get out for a short ride today with the Twofish bottle cage attached to the seat tube. All worked well - the cage (and bottle) stayed secure. I do have a quick release rear mudguard (Topeak) fitted and the rear of the bottle cage was resting on the Q/R so that may have provided extra security on the seat post.

The bottle did not interfere with my pedaling (my lack of fitness did!) and the whole system worked well. I don't drink while on the move and always stop to take a sip of liquid so do not know if it would be easy to access the bottle while in the saddle. I tucked the long strap of Velcro behind the bottle but may cut it shorter once I'm sure where I want the cage to sit.