You correct here. The problem as Raytheon has indicated is that there is no way to calibrate the data without the St60/70 display/control head, other that that it should work. Being a EE this is a very easy problem to solve with passive components (resistors, capacitors). The direction indicator is nothing more than a variable resistor, and the speed is a rotating magnet, which induces a EMF ( Electro Motive Force) into a special semiconductor device (a Hall Effect Transistor) that is sensitive to magnetic fields. So, as it rotates it creates pulses. The pulses get averaged out in a RC (resistor, capacitor) network to create a voltage which is read with a A to D (Analog to Digital Converter) into the control head, the same way digital speedometers work in cars.
I'm still not convinced that the C70w will display wind info, the manual is not very clear on this. So that will be the rub on this one. I can do the tweaking in the analog world to calibrate the vane, but is it worth the $139 dollars difference than just buying the farm? Anyway it is always fun to out fox the eng/marketting people at Raymarine!