Hi Luiz,
"If you want to extend the jib foot down to the spinaker pole, just add a vertical strut and it's done. The strut will also reduce the free length of the pole, so it will serve two structural purposes. Seems quite simple, light and efficient to me".

You are 'right on' again; you understand correctly.
The equation for 'max allowable load' in a tall slender column is: P = Phi**2 x E x I / L**2 . This is a stability equation and has to do with 'buckling failures'. The import thing to notice here is that the moment of inertia, I, is in the numerator and the the unsupported length, L, is in denominator. For spinnaker pole schemes without support at the pole mid point, L is the total length of the pole. If the pole is stabalized at pole midpoint so that it can't move vertically and it can't move sideways, then the L term in the equation is L/2 which is then squared. This makes the denominator 1/4 as large as it is for the unsupported pole and therefore the I term in the numerator can be 1/4th as large, a much smaller diameter pole, and lighter weight also.
On the SC20 at first I used a 3/4inch diameter SS thin walled tube 2ft long to connect the mid pole point to the apex of the forestay bridle wires. This short tube buckled and I had to go up to a 1 inch diameter SS tube. The system really works and my spinnaker pole is much lighter weight than others of similiar length.
Bill