Hi there,
I am not entirely sure how to answer this question because the coaxial capacitance of the insulated wire is due to dimensional and other physical issues such as variance in the thickness of the dielectric, changes in the specific gravity of the dielectric, changes in the concentricity of the dielectric and of course changes in the conductor diameter itself say by stretching. On a production basis and in an industrial plant, coaxial capacitance can be measured remotely by continuously immersing the wire in water and reading the coaxial capacitance to ground but I doubt that you want to get the taped wire wet. I also imagine that contact measurements are not allowed.
This then leads to the accurate measurement of the dimensions (diameters, conductor out of round, and tape or dielectric concentricity.) of the continuously manufactured wire by laser before and after insulating and then by inferring the capacitance from those measurements. The management of the tape overlap dimension as a constant will be a critical as will any wrinkles in the applied tape. This should all be fairly straightforward because you are dealing with a tape rather than an extruded dielectric.
We also believe that you are already measuring the structural return loss (SRL) so you must have a pretty good handle on things.
The companies you should be talking with are:
Beta LaserMike at
www.betalasermike.com/ ,
Sikora AG at
www.sikora-usa.com/ , and
ZUMBACH Electronic AG at
www.zumbach.com/e/index.htm
Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com 
