You can use curves as hair guides. Set their maximum influence to create a small "sphere" around their base which influences the hair system, which will allow short hairs, and under the curve's influence only, long hairs.
Than you can use shape keys to animate the curve. I'm not sure if you can use an armature but that may also be an option through some other round about method.
Here is a step by step, sorry there is no screen shots.
select an object and create a particle system. Select "Hair" as
the type. You will see some strands.
create a bezier curve (shift+a -> curve -> bezier curve)
this is the step that may have been unclear sorry. Select the
curve, and go to the Physics tab, and click "Force Field". In the
properties below, under "Type" change from "Force" to "Curve Guide".
There will be 2 circles, a dotted and solid at each point. The solid
sphere is the root of the guide.
Check the "Use Max" checkbox. This will change the root to a dashed
sphere. Set the maximum distance as desired. It will change the size
of that sphere, and any hairs inside of it, will be influenced by
the hair guide.
Now changing the hair system in particle settings will affect the normal parts, and the hair guide fill affect some specific parts.