Press Fit Pressure
Assuming that you can (or have already) calculated the pressure between the head and the tube - there is a section on cylinders under pressure in 'Roark's Formulas for Stress and Strain', look under autofrettage.
This pressure is constant around the contact diameter, so take the area of engagement between the head and the tube ie. contact diameter x Pi x contact length and multiply it by the pressure already calculated, to give the total force. Now multiply by your coefficient of friction to give the friction force and finally, multiply by the contact radius to give you the torque required.
It seems fairly straightforward, or have I missed the point here?
Do bear in mind that COF's are a bit hit and miss, and also it will take more torque at breakaway (to get it started moving), than it will to keep it moving.