The issue with doing that, is that the hammer cost itself determines whether or not a unit can be built. So by giving GP a hammer cost, you'd be allowing people to build them. You could probably finagle it by putting other restrictions on the units in question (tech prereqs that can never be met, or something similar), but it's not an ideal solution.
FYI, Super Spies 1.2 is out now, and includes its own form of unit assassination. The main differences between it and the base game functionality is that any unit can be targeted, even in friendly or neutral territroy, but it must be alone. There is no hammer cost restriction so GP are targettable, but both costs and chances of success are directly modified by unit level.