It will have to be phase inverted to both remaining pickups, so you have to check how each Fender PU phase is as they're usually phase inverted (don't know who is out of phase though). But to work as Alembic does, dummy strengh have to match PU's strengh.
As far as I remember, in Alembic Series the dummy PU have two coils with separated gain control to match noise amplitude from each pickup. Seems to be difficult to have the same magnetic strengh on each coil (there are so many factors, as winding number, wire gauge and how magnet was made), so their signal (and induced noise) tend to be slightly different and thats what can affect how much cancel you will get.
Other thing to consider is that the dummy in Alembic have two coils, one per PU, and that's why you can use one single PU and sum keeps silent. If you'll use just one single dummy coil, your strat will have to use both PU on same phase and always on. If you want to use just one of them sometime, you will need two coils on this dummy and they have to match noise level from each PUs. Not easy to achieve, and that seems to be why Ron added gain adjustment to each coil in the dummy.
This is not easy subject and I hope my lousy english doesn't mess with it some more...