well, from a rigging perspective if you want something to look midway between two points, you have to take the two points and find the average point in between them... this is me thinking off the top of my head and i'm not sure how well to do it in playmaker... but here goes nothing.
first, if you can get the rotational information from the preceeding bone object or have a game-object that's oriented the same as the head bone but parented to the neck bone (so you have something to extract the default information from.) i would probably add in another game-object parented to the neck bone with the look-at script attached to be the second object to extract the rotation data of...
from there, i am not sure which math-like actions to use but if you can take both those objects, average out the data (so, maybe add the rotation in each of the three axis, divide by two and recompile that data back into a vector3 variable) and tell the head bone itself to orient itself with the rotational data in this new vector3 variable you've just made might do it... if you're having all this happen every frame, it should (theoretically) give you real-time data... and with the head having the variable information fed to it and orienting it, it can be (i think) simple matter of setting up a system where you tell it "use this" and "don't use this." with either global variables fired off as events from whatever FSM you want to control it with.
again, though, i dont' know how this would be do-able in playmaker... but it's kinda what i'd do if i were rigging it and the systems aren't terribly different in that respect as they use bone-objects for extracting the transform information.