I'd imagine that they HDS would just take a light reading using an IS with the head assembled. I'm not sure. It's probably a lot more complicated than I imagine.
The point of taking a light reading is to get the output established. Now it won't be the correct one, so the light now has to be calibrated (eg, 172 lumens, calibrate electronics for it to go down to 170 lumens w/ longer battery life).
That is not possible with head assembled. Unless of course, you disassemble it after that, put it together and the ship it off.