I can't see it being possible TBH. Whereas the early cable/hydraulic MTX casings still had the castings for the speedo drive (as with the rod/mechanical) just not machined, the next series of boxes (with a lower oil volume) had all these legacy parts deleted.

As the gearbox front/bellhousing your using is later still I can't see there being anything to machine (that would be difficult enough anyway) for a mechanical speedo drive.

I don't know if they have changed it further, but on all the Mondeo Mk2, Focus boxes they replaced the speedo worm gear on the diff with a metal/plastic pickup wheel for the hall-effect sensor. The MT285 (ST170) box doesn't have any pickup wheel; the sensor just 'reads' the teeth on the crownwheel.

Martin