I honestly have not tested on act bosses. Are you sure that is true? I ask because I know they can be frozen by other attacks and skills. Well, I at least Mephy can be frozen, and I think Andy and Duriel as well. I'll test right now... with skills, that is (don't have a Freeze Pally or Doom).
The "chilling" effect from Holy Freeze iirc is not a cold effect, it turns your character blue and slows you down as if you were hit by ice but the game code is not ice damage/effect. I believe it works the same way as Cleglaws does.
As far as act bosses being chilled/frozen....I don't believe you can freeze any of them (I don't think you can even freeze normal unique monsters). Where freezeing them actualy stops them dead in there tracks. But you can Chill just about any monster, as long as it's not cold immune, I believe this includes act bosses. Whether Holy Freeze works against act bosses I"m can't remember. The hammerdin I'm playing currently has a HF merc but everything dies too quickly for me to really care if its been slowed and I havn't run act bosses for a few weeks.
Yeah, act bosses can be chilled by cold attacks, but they usually don't last more than a second, so unless you have something like a Hurricane up, don't expect to get much use of cold spells to slow them down.
Well, Holy Freeze changed in 1.10, and it now does actual cold damage. I'm not sure how this works against act bosses, but I would assume it could freeze (when I say freeze I mean turn blue and slow down and deal cold damage, as true freezing doesn't occur often) act bosses just like any other cold attack.
If I'm not mistaking.....Holy freeze did change in 1.10 to add some periodic cold damage, but it's still a slow effect that slows them not the cold. The periodic cold damage gives you a chance to shatter most monsters though. I'll check latter to make sure that it hasn't been completly converted to a cold effect. I'll just find some cold immunes and see if they turn blue/move slow with my mercs aura arround.